f2fs: find parent dentry correctly

If dotdot directory is corrupted, its slot may be ocupied by another
file. In this case, dentry[1] is not the parent directory. Rename and
cross-rename will update the inode in dentry[1] incorrectly.   This
patch finds dotdot dentry by name.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
[Jaegeuk Kim: remove wron bug_on]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>