libceph: fix return value check in alloc_msg_with_page_vector()

In case of error, the function ceph_alloc_page_vector() returns
ERR_PTR() and never returns NULL. The NULL test in the return value
check should be replaced with IS_ERR().

Fixes: 19079203 ('libceph: support for sending notifies')
Signed-off-by: NWei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

net/ceph/osd_client.c

@@ -4220,7 +4220,7 @@ static struct ceph_msg *alloc_msg_with_page_vector(struct ceph_msg_header *hdr)
 
 		pages = ceph_alloc_page_vector(calc_pages_for(0, data_len),
 					       GFP_NOIO);
-		if (!pages) {
+		if (IS_ERR(pages)) {
 			ceph_msg_put(m);
 			return NULL;
 		}