xfs: random buffer write failure errortag

xfs: random buffer write failure errortag

Source kernel commit: 7376d74547344598008d00419eae0caa5f50f4f0

Introduce an error tag to randomly fail async buffer writes. This is
primarily to facilitate testing of the XFS error configuration
mechanism.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>