libceph: osdc->req_mempool should be backed by a slab pool

ceph_osd_request_cache was introduced a long time ago.  Also, osd_req
is about to get a flexible array member, which ceph_osd_request_cache
is going to be aware of.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/osd_client.c

@@ -2648,8 +2648,8 @@ int ceph_osdc_init(struct ceph_osd_client *osdc, struct ceph_client *client)
 	    round_jiffies_relative(osdc->client->options->osd_idle_ttl));
 
 	err = -ENOMEM;
-	osdc->req_mempool = mempool_create_kmalloc_pool(10,
-					sizeof(struct ceph_osd_request));
+	osdc->req_mempool = mempool_create_slab_pool(10,
+						     ceph_osd_request_cache);
 	if (!osdc->req_mempool)
 		goto out;