Dave Jones found a warning from kasan in setup_cluster_bitmaps()

==================================================================
BUG: KASAN: stack-out-of-bounds in setup_cluster_bitmap+0xc4/0x5a0 at
addr ffff88039bef6828
Read of size 8 by task nfsd/1009
page:ffffea000e6fbd80 count:0 mapcount:0 mapping:          (null)
index:0x0
flags: 0x8000000000000000()
page dumped because: kasan: bad access detected
CPU: 1 PID: 1009 Comm: nfsd Tainted: G        W
4.4.0-rc3-backup-debug+ #1
 ffff880065647b50 000000006bb712c2 ffff88039bef6640 ffffffffa680a43e
 0000004559c00000 ffff88039bef66c8 ffffffffa62638d1 ffffffffa61121c0
 ffff8803a5769de8 0000000000000296 ffff8803a5769df0 0000000000046280
Call Trace:
 [<ffffffffa680a43e>] dump_stack+0x4b/0x6d
 [<ffffffffa62638d1>] kasan_report_error+0x501/0x520
 [<ffffffffa61121c0>] ? debug_show_all_locks+0x1e0/0x1e0
 [<ffffffffa6263948>] kasan_report+0x58/0x60
 [<ffffffffa6814b00>] ? rb_last+0x10/0x40
 [<ffffffffa66f8af4>] ? setup_cluster_bitmap+0xc4/0x5a0
 [<ffffffffa6262ead>] __asan_load8+0x5d/0x70
 [<ffffffffa66f8af4>] setup_cluster_bitmap+0xc4/0x5a0
 [<ffffffffa66f675a>] ? setup_cluster_no_bitmap+0x6a/0x400
 [<ffffffffa66fcd16>] btrfs_find_space_cluster+0x4b6/0x640
 [<ffffffffa66fc860>] ? btrfs_alloc_from_cluster+0x4e0/0x4e0
 [<ffffffffa66fc36e>] ? btrfs_return_cluster_to_free_space+0x9e/0xb0
 [<ffffffffa702dc37>] ? _raw_spin_unlock+0x27/0x40
 [<ffffffffa666a1a1>] find_free_extent+0xba1/0x1520

Andrey noticed this was because we were doing list_first_entry on a list
that might be empty.  Rework the tests a bit so we don't do that.
Signed-off-by: Chris Mason <clm@fb.com>
Reprorted-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Reported-by: Dave Jones <dsj@fb.com>