mpage: Provide variant of mpage_writepages() with own optional folio handler

mpage: Provide variant of mpage_writepages() with own optional folio handler

Some filesystems need to treat some folios specially (for example for
inodes with inline data). Doing the handling in their .writepages method
in a race-free manner results in duplicating some of the writeback
internals. So provide generalized version of mpage_writepages() that
allows filesystem to provide a handler called for each folio which can
handle the folio in a special way.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260326140635.15895-3-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>