xfs: add knobs for testing iomap write race fixes This series is a followup to Dave Chinner's series entitled "xfs, iomap: fix data corruption due to stale cached iomaps". The two patches here add debugging knobs to introduce artificial delays into the pagecache write and writeback code to facilitate testing of the iomap invalidation code. New tracepoints are also introduced so that fstests can look for the invalidations. Signed-off-by: Darrick J. Wong <djwong@kernel.org>