ceph: checking for IS_ERR instead of NULL

ceph_osdc_alloc_request() returns NULL on error, it never returns error
pointers.

Fixes: 5be0389dac66 ('ceph: re-send AIO write request when getting -EOLDSNAP error')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/file.c

@@ -698,8 +698,8 @@ static void ceph_aio_retry_work(struct work_struct *work)
 
 	req = ceph_osdc_alloc_request(orig_req->r_osdc, snapc, 2,
 			false, GFP_NOFS);
-	if (IS_ERR(req)) {
-		ret = PTR_ERR(req);
+	if (!req) {
+		ret = -ENOMEM;
 		req = orig_req;
 		goto out;
 	}