libceph: potential NULL dereference in ceph_msg_data_create()

If kmem_cache_zalloc() returns NULL then the INIT_LIST_HEAD(&data->links);
will Oops.  The callers aren't really prepared for NULL returns so it
doesn't make a lot of difference in real life.

Fixes: 5240d9f9 ("libceph: replace message data pointer with list")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/messenger.c

@@ -3203,7 +3203,9 @@ static struct ceph_msg_data *ceph_msg_data_create(enum ceph_msg_data_type type)
 		return NULL;
 
 	data = kmem_cache_zalloc(ceph_msg_data_cache, GFP_NOFS);
-	if (data)
+	if (!data)
+		return NULL;
+
 	data->type = type;
 	INIT_LIST_HEAD(&data->links);