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>
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