xfs: Remove kmem_zone_free() wrapper

We can remove it now, without needing to rework the KM_ flags.

Use kmem_cache_free() directly.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>