Btrfs: check return value of ulist_alloc() properly

ulist_alloc() has the possibility of returning NULL.
So, it is necessary to check the return value.
Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>

fs/btrfs/qgroup.c

@@ -1469,6 +1469,10 @@ int btrfs_qgroup_reserve(struct btrfs_root *root, u64 num_bytes)
 	 * be exceeded
 	 */
 	ulist = ulist_alloc(GFP_ATOMIC);
+	if (!ulist) {
+		ret = -ENOMEM;
+		goto out;
+	}
 	ulist_add(ulist, qgroup->qgroupid, (uintptr_t)qgroup, GFP_ATOMIC);
 	ULIST_ITER_INIT(&uiter);
 	while ((unode = ulist_next(ulist, &uiter))) {
@@ -1541,6 +1545,10 @@ void btrfs_qgroup_free(struct btrfs_root *root, u64 num_bytes)
 		goto out;
 
 	ulist = ulist_alloc(GFP_ATOMIC);
+	if (!ulist) {
+		btrfs_std_error(fs_info, -ENOMEM);
+		goto out;
+	}
 	ulist_add(ulist, qgroup->qgroupid, (uintptr_t)qgroup, GFP_ATOMIC);
 	ULIST_ITER_INIT(&uiter);
 	while ((unode = ulist_next(ulist, &uiter))) {