bcachefs: Limit bios in writepages path to 256M

This works around a bug where bio_full() doesn't check for
bio->bi_iter.bi_size overflowing - and, we don't really want to build
bios that are that big anyways.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-io.c

@@ -1151,6 +1151,7 @@ static int __bch2_writepage(struct folio *folio,
 		if (w->io &&
 		    (w->io->op.res.nr_replicas != nr_replicas_this_write ||
 		     bio_full(&w->io->op.wbio.bio, PAGE_SIZE) ||
+		     w->io->op.wbio.bio.bi_iter.bi_size >= (256U << 20) ||
 		     bio_end_sector(&w->io->op.wbio.bio) != sector))
 			bch2_writepage_do_io(w);