xfs: Remove slab init wrappers

Remove kmem_zone_init() and kmem_zone_init_flags() together with their
specific KM_* to SLAB_* flag wrappers.

Use kmem_cache_create() 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>