xfs: add debug knob to slow down writeback for fun

Add a new error injection knob so that we can arbitrarily slow down
writeback 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
745b3f76 ("xfs: maintain a sequence count for inode fork
manipulations").

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