Btrfs: return errno if possible when we fail to allocate memory

We need to set return value explicitly, otherwise we'll lose the error
value.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/inode.c

@@ -714,8 +714,10 @@ retry:
 					async_extent->ram_size - 1, 0);
 
 		em = alloc_extent_map();
-		if (!em)
+		if (!em) {
+			ret = -ENOMEM;
 			goto out_free_reserve;
+		}
 		em->start = async_extent->start;
 		em->len = async_extent->ram_size;
 		em->orig_start = em->start;
@@ -922,8 +924,10 @@ static noinline int __cow_file_range(struct btrfs_trans_handle *trans,
 		}
 
 		em = alloc_extent_map();
-		if (!em)
+		if (!em) {
+			ret = -ENOMEM;
 			goto out_reserve;
+		}
 		em->start = start;
 		em->orig_start = em->start;
 		ram_size = ins.offset;