xfs/051: test buffer use after free race on I/O failure in XFS log recovery

A buffer use after free race was discovered in the XFS log recovery
codepath if I/O failures occur during recovery. The I/O submission path
can abort the mount and release the only reference held on some buffers
before I/O completion processing (e.g., async workqueue processing)
might have completed. Badness ensues if the I/O completion path
subsequently attempts to access said buffers.

The test manufactures the race by forcing all writes to fail (via
dm-flakey) after a fixed period of time. A delay is inserted into the
mount codepath to synchronize write failures with log recovery.

Credit for discovery of the race and definition of the reproducible test
case goes to Alex Lyakas.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reported-by: Alex Lyakas <alex@zadarastorage.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/xfs/051.out

@@ -0,0 +1,2 @@
+QA output created by 051
+Silence is golden.