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

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.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 8c26c4e2694a ("nilfs2: fix issue with flush kernel thread after remount in RO mode because of driver's internal error or metadata corruption")
Signed-off-by: Ryusuke Konishi <[email protected]>
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=d6ca2daf692c7a82f959
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>