I hit the following oops out of the blue while testing with flexfiles:

BUG: unable to handle kernel NULL pointer dereference at 00000000000000e8
IP: [<ffffffffa048f6b8>] nfs4_ff_find_or_create_ds_client+0x48/0x50 [nfs_layout_flexfiles]
PGD 44031067 PUD 5062d067 PMD 0
Oops: 0000 [#1] SMP
Modules linked in: nfsv3 nfs_layout_flexfiles tun rpcsec_gss_krb5 nfsv4 dns_resolver nfs fscache dcdbas nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip6t_rpfilter ip6t_REJECT nf_reject_ipv6 xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw bonding ipmi_devintf ipmi_msghandler snd_hda_codec_generic virtio_balloon ppdev snd_hda_intel snd_hda_controller snd_hda_codec iosf_mbi crct10dif_pclmul crc32_pclmul ghash_clmulni_intel snd_hda_core parport_pc snd_hwdep parport snd_seq snd_seq_device snd_pcm snd_timer acpi_cpufreq
 snd soundcore i2c_piix4 xfs libcrc32c joydev virtio_net virtio_console qxl drm_kms_helper ttm crc32c_intel drm virtio_pci serio_raw ata_generic virtio_ring virtio pata_acpi
CPU: 0 PID: 19138 Comm: test5 Not tainted 4.1.9-100.pd.90.el7.x86_64 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.2-20150714_191134- 04/01/2014
task: ffff88007b70cf00 ti: ffff88004cc44000 task.ti: ffff88004cc44000
RIP: 0010:[<ffffffffa048f6b8>]  [<ffffffffa048f6b8>] nfs4_ff_find_or_create_ds_client+0x48/0x50 [nfs_layout_flexfiles]
RSP: 0018:ffff88004cc47890  EFLAGS: 00010246
RAX: 0000000000000003 RBX: ffff880050932300 RCX: ffff88006978f488
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88003e0e8540
RBP: ffff88004cc47908 R08: 0000000000000000 R09: 0000000000000000
R10: ffff88007ff8c758 R11: 0000000000000005 R12: ffff88003e0e8540
R13: 0000000000000000 R14: ffff88006978f488 R15: ffff88004431cc80
FS:  00007fea40c7c740(0000) GS:ffff88007fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000000000e8 CR3: 0000000044318000 CR4: 00000000000406f0
Stack:
 ffffffffa048c934 ffff880050932310 0000000100000001 ffff88006978f510
 ffff88006978f3c8 ffff88003e56cd90 ffff88004cc479d0 00000020a052aff0
 000000000004b000 ffff88004cc47908 ffff880050932300 ffff88004cc479d0
Call Trace:
 [<ffffffffa048c934>] ? ff_layout_write_pagelist+0x64/0x220 [nfs_layout_flexfiles]
 [<ffffffffa057a3bf>] pnfs_generic_pg_writepages+0xaf/0x1b0 [nfsv4]
 [<ffffffffa051ab57>] nfs_pageio_doio+0x27/0x60 [nfs]
 [<ffffffffa051bfe4>] nfs_pageio_complete_mirror+0x54/0xa0 [nfs]
 [<ffffffffa051c7ad>] nfs_pageio_complete+0x2d/0x90 [nfs]
 [<ffffffffa052032d>] nfs_writepage_locked+0x8d/0xe0 [nfs]
 [<ffffffff811e4630>] ? page_referenced_one+0x1a0/0x1a0
 [<ffffffffa05210e7>] nfs_wb_single_page+0xf7/0x190 [nfs]
 [<ffffffffa05108d1>] nfs_launder_page+0x41/0x90 [nfs]
 [<ffffffff811b8930>] invalidate_inode_pages2_range+0x340/0x3a0
 [<ffffffff811b89a7>] invalidate_inode_pages2+0x17/0x20
 [<ffffffffa0513e1e>] nfs_release+0x9e/0xb0 [nfs]
 [<ffffffffa050fa1d>] nfs_file_release+0x3d/0x60 [nfs]
 [<ffffffff8122481c>] __fput+0xdc/0x1e0
 [<ffffffff8122496e>] ____fput+0xe/0x10
 [<ffffffff810bde67>] task_work_run+0xa7/0xe0
 [<ffffffff810af735>] get_signal+0x565/0x600
 [<ffffffff811a9815>] ? __filemap_fdatawrite_range+0x65/0x90
 [<ffffffff810144a7>] do_signal+0x37/0x730
 [<ffffffffa0569921>] ? nfs4_file_fsync+0x81/0x150 [nfsv4]
 [<ffffffff81254dbb>] ? vfs_fsync_range+0x3b/0xb0
 [<ffffffff811446a6>] ? __audit_syscall_exit+0x1e6/0x280
 [<ffffffff81014bff>] do_notify_resume+0x5f/0xa0
 [<ffffffff8178ec3c>] int_signal+0x12/0x17
Code: 48 8b 40 70 8b 00 83 f8 03 74 20 83 f8 04 75 13 55 48 89 ce 48 89 d7 48 89 e5 e8 14 0f 0e 00 5d c3 66 90 0f 0b 66 0f 1f 44 00 00 <48> 8b 82 e8 00 00 00 c3 66 66 66 66 90 55 48 89 e5 41 57 41 56
RIP  [<ffffffffa048f6b8>] nfs4_ff_find_or_create_ds_client+0x48/0x50 [nfs_layout_flexfiles]
 RSP <ffff88004cc47890>
CR2: 00000000000000e8

When the DS connection attempt fails, nfs4_ff_layout_prepare_ds marks it
for the error but then just returns the ds as if it were usable. The
comments though say:

  /* Upon return, either ds is connected, or ds is NULL */

Ensure that we set the return pointer to NULL in the event that the
connection attempt fails.
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>