xfs: regression test for writeback corruption bug

xfs: regression test for writeback corruption bug

This is a regression test for a data corruption bug that existed in XFS'
copy on write code between 4.9 and 4.19.  The root cause is a
concurrency bug wherein we would drop ILOCK_SHARED after querying the
CoW fork in xfs_map_cow and retake it before querying the data fork in
xfs_map_blocks.  See the test description for a lot more details.

Cc: Wengang Wang <wen.gang.wang@oracle.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>