f2fs: make trace enter and end in pairs for unlink

In the f2fs_unlink we do not add trace end for some
error paths, just add.

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