btrfs: fix extent map use-after-free when handling missing device in read_one_chunk

Store the error code before freeing the extent_map. Though it's
reference counted structure, in that function it's the first and last
allocation so this would lead to a potential use-after-free.

The error can happen eg. when chunk is stored on a missing device and
the degraded mount option is missing.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=216721Reported-by: eriri <1527030098@qq.com>
Fixes: adfb69af ("btrfs: add_missing_dev() should return the actual error")
CC: stable@vger.kernel.org # 4.9+
Signed-off-by: void0red <void0red@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -6976,8 +6976,9 @@ static int read_one_chunk(struct btrfs_key *key, struct extent_buffer *leaf,
 			map->stripes[i].dev = handle_missing_device(fs_info,
 								    devid, uuid);
 			if (IS_ERR(map->stripes[i].dev)) {
+				ret = PTR_ERR(map->stripes[i].dev);
 				free_extent_map(em);
-				return PTR_ERR(map->stripes[i].dev);
+				return ret;
 			}
 		}