nilfs2: fix kernel bug due to missing clearing of buffer delay flag

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

nilfs2: fix kernel bug due to missing clearing of buffer delay flag

Syzbot reported that after nilfs2 reads a corrupted file system image

and degrades to read-only, the BUG_ON check for the buffer delay flag

in submit_bh_wbc() may fail, causing a kernel bug.

This is because the buffer delay flag is not cleared when clearing the

buffer state flags to discard a page/folio or a buffer head. So, fix

this.

This became necessary when the use of nilfs2's own page clear routine

was expanded. This state inconsistency does not occur if the buffer

is written normally by log writing.