libceph: fix ceph_osdc_alloc_request error checks

ceph_osdc_alloc_request returns NULL on failure.
Signed-off-by: Sage Weil <sage@newdream.net>

drivers/block/rbd.c

@@ -777,9 +777,9 @@ static int rbd_do_request(struct request *rq,
 				      ops,
 				      false,
 				      GFP_NOIO, pages, bio);
-	if (IS_ERR(req)) {
+	if (!req) {
 		up_read(&header->snap_rwsem);
-		ret = PTR_ERR(req);
+		ret = -ENOMEM;
 		goto done_pages;
 	}
 

net/ceph/osd_client.c

@@ -470,8 +470,8 @@ struct ceph_osd_request *ceph_osdc_new_request(struct ceph_osd_client *osdc,
 					 snapc, ops,
 					 use_mempool,
 					 GFP_NOFS, NULL, NULL);
-	if (IS_ERR(req))
-		return req;
+	if (!req)
+		return NULL;
 
 	/* calculate max write size */
 	calc_layout(osdc, vino, layout, off, plen, req, ops);