nilfs2: fix kernel bug on rename operation of broken directory

In the Linux kernel, the following vulnerability has been resolved:

nilfs2: fix kernel bug on rename operation of broken directory

Syzbot reported that in rename directory operation on broken directory on nilfs2, __block_write_begin_int() called to prepare block write may fail BUG_ON check for access exceeding the folio/page size.

This is because nilfs_dotdot(), which gets parent directory reference entry ("..") of the directory to be moved or renamed, does not check consistency enough, and may return location exceeding folio/page size for broken directories.

Fix this issue by checking required directory entries ("." and "..") in the first chunk of the directory in nilfs_dotdot().