xfs/170 on a filesystem with su=128k,sw=4 produces this splat:

BUG: kernel NULL pointer dereference, address: 0000000000000010
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 0 P4D 0
Oops: 0002 [#1] PREEMPT SMP
CPU: 1 PID: 4022907 Comm: dd Tainted: G        W          6.3.0-xfsx #2 6ebeeffbe9577d32
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20171121_152543-x86-ol7-bu
RIP: 0010:xfs_perag_rele+0x10/0x70 [xfs]
RSP: 0018:ffffc90001e43858 EFLAGS: 00010217
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000100
RDX: ffffffffa054e717 RSI: 0000000000000005 RDI: 0000000000000000
RBP: ffff888194eea000 R08: 0000000000000000 R09: 0000000000000037
R10: ffff888100ac1cb0 R11: 0000000000000018 R12: 0000000000000000
R13: ffffc90001e43a38 R14: ffff888194eea000 R15: ffff888194eea000
FS:  00007f93d1a0e740(0000) GS:ffff88843fc80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000010 CR3: 000000018a34f000 CR4: 00000000003506e0
Call Trace:
 <TASK>
 xfs_bmap_btalloc+0x1a7/0x5d0 [xfs f85291d6841cbb3dc740083f1f331c0327394518]
 xfs_bmapi_allocate+0xee/0x470 [xfs f85291d6841cbb3dc740083f1f331c0327394518]
 xfs_bmapi_write+0x539/0x9e0 [xfs f85291d6841cbb3dc740083f1f331c0327394518]
 xfs_iomap_write_direct+0x1bb/0x2b0 [xfs f85291d6841cbb3dc740083f1f331c0327394518]
 xfs_direct_write_iomap_begin+0x51c/0x710 [xfs f85291d6841cbb3dc740083f1f331c0327394518]
 iomap_iter+0x132/0x2f0
 __iomap_dio_rw+0x2f8/0x840
 iomap_dio_rw+0xe/0x30
 xfs_file_dio_write_aligned+0xad/0x180 [xfs f85291d6841cbb3dc740083f1f331c0327394518]
 xfs_file_write_iter+0xfb/0x190 [xfs f85291d6841cbb3dc740083f1f331c0327394518]
 vfs_write+0x2eb/0x410
 ksys_write+0x65/0xe0
 do_syscall_64+0x2b/0x80

This crash occurs under the "out_low_space" label.  We grabbed a perag
reference, passed it via args->pag into xfs_bmap_btalloc_at_eof, and
afterwards args->pag is NULL.  Fix the second function not to clobber
args->pag if the caller had passed one in.

Fixes: 85843327 ("xfs: factor xfs_bmap_btalloc()")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>