xfs/179: modify test to trigger refcount update bugs

xfs/179: modify test to trigger refcount update bugs

Upon enabling fsdax + reflink for XFS, this test began to report
refcount metadata corruptions after being run.  Specifically, xfs_repair
noticed single-block refcount records that could be combined but had not
been.

The root cause of this is improper MAXREFCOUNT edge case handling in
xfs_refcount_merge_extents.  When we're trying to find candidates for a
record merge, we compute the refcount of the merged record, but without
accounting for the fact that once a record hits rc_refcount ==
MAXREFCOUNT, it is pinned that way forever.

Adjust this test to use a sub-filesize write for one of the COW writes,
because this is how we force the extent merge code to run.

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