xfs: add debug knob to slow down write for fun

xfs: add debug knob to slow down write for fun

Source kernel commit: 254e3459285cbf2174350bbc0051e475e1bc5196

Add a new error injection knob so that we can arbitrarily slow down
pagecache writes to test for race conditions and aberrant reclaim
behavior if the writeback mechanisms are slow to issue writeback.  This
will enable functional testing for the ifork sequence counters
introduced in commit 304a68b9c63b ("xfs: use iomap_valid method to
detect stale cached iomaps") that fixes write racing with reclaim
writeback.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>