xfs: regression test for writes racing with reclaim writeback

xfs: regression test for writes racing with reclaim writeback

This test uses the new write delay debug knobs to set up a slow-moving
write racing with writeback of an unwritten block at the end of the
file range targetted by the slow write.  The test succeeds if the file
does not end up corrupt and if the ftrace log captures a message about
the revalidation occurring.

NOTE: I'm not convinced that madvise actually causes the page to be
removed from the pagecache, which means that this is effectively a
functional test for the invalidation, not a corruption reproducer.
On the other hand, the functional test can be done quickly.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>