Btrfs: fix extent map leak during fallocate error path

BugLink: https://bugs.launchpad.net/bugs/1822579

If the call to btrfs_qgroup_reserve_data() failed, we were leaking an
extent map structure. The failure can happen either due to an -ENOMEM
condition or, when quotas are enabled, due to -EDQUOT for example.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
(backported from commit be2d253c)
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

fs/btrfs/file.c

@@ -2763,8 +2763,10 @@ static long btrfs_fallocate(struct file *file, int mode,
 			}
 			ret = btrfs_qgroup_reserve_data(inode, cur_offset,
 					last_byte - cur_offset);
-			if (ret < 0)
+			if (ret < 0) {
+				free_extent_map(em);
 				break;
+			}
 		}
 		free_extent_map(em);
 		cur_offset = last_byte;