kfence_shutdown_cache() is called under slab_mutex when the cache is
destroyed synchronously, and outside slab_mutex during the delayed
destruction of SLAB_TYPESAFE_BY_RCU caches.

It seems it should always be safe to call it outside of slab_mutex so we
can just move the call to kmem_cache_release(), which is called outside.
Reviewed-by: Jann Horn <jannh@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>