reflink: test unlinking a huge extent with a lot of refcount adjustments

reflink: test unlinking a huge extent with a lot of refcount adjustments

Test a regression in XFS where we blow out a transaction reservation if
we create a big file, share every other block, and delete the first
file.  There's nothing particularly fs-specific about this stress test,
so put it in generic.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>