nfsd: Simplify the allocation of slab caches in nfsd4_init_slabs

Use the new KMEM_CACHE() macro instead of direct kmem_cache_create
to simplify the creation of SLAB caches.
Make the code cleaner and more readable.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>