f2fs: convert inline_dir early before starting rename

If we hit an error during rename, we'll get two dentries in different
directories.

Chao adds to check the room in inline_dir which can avoid needless
inversion. This should be done by inode_lock(&old_dir).

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>