Make vn_cache|vn_file_cache kmem caches

Both of these caches were previously allowed to be either a
vmem or kmem cache based on the size of the object involved.
Since we know the object won't be to large and performce is
much better for a kmem cache for them to be kmem backed.

module/spl/spl-vnode.c

@@ -795,13 +795,13 @@ vn_init(void)
 				     sizeof(struct vnode), 64,
 	                             vn_cache_constructor,
 				     vn_cache_destructor,
-				     NULL, NULL, NULL, 0);
+				     NULL, NULL, NULL, KMC_KMEM);
 
 	vn_file_cache = kmem_cache_create("spl_vn_file_cache",
 					  sizeof(file_t), 64,
 				          vn_file_cache_constructor,
 				          vn_file_cache_destructor,
-				          NULL, NULL, NULL, 0);
+				          NULL, NULL, NULL, KMC_KMEM);
 	SRETURN(0);
 } /* vn_init() */