nilfs2: fix kernel bug due to missing clearing of checked flag

In the Linux kernel, the following vulnerability has been resolved: nilfs2: fix kernel bug due to missing clearing of checked flag Syzbot reported that in directory operations after nilfs2 detects filesystem corruption and degrades to read-only, __block_write_begin_int(), which is called to prepare block writes, may fail the BUG_ON check for accesses exceeding the folio/page size, triggering a kernel bug. This was found to be because the "checked" flag of a page/folio was not cleared when it was discarded by nilfs2's own routine, which causes the sanity check of directory entries to be skipped when the directory page/folio is reloaded. So, fix that. This was necessary when the use of nilfs2's own page discard routine was applied to more than just metadata files.