block: fix "check bi_size overflow before merge"

This partially reverts commit e3a5d8e3.

Commit e3a5d8e3 ("check bi_size overflow before merge") adds a bio_full
check to __bio_try_merge_page.  This will cause __bio_try_merge_page to fail
when the last bi_io_vec has been reached.  Instead, what we want here is only
the bi_size overflow check.

Fixes: e3a5d8e3 ("block: check bi_size overflow before merge")
Cc: stable@vger.kernel.org # v5.4+
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -754,10 +754,12 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
 	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
 		return false;
 
-	if (bio->bi_vcnt > 0 && !bio_full(bio, len)) {
+	if (bio->bi_vcnt > 0) {
 		struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
 
 		if (page_is_mergeable(bv, page, len, off, same_page)) {
+			if (bio->bi_iter.bi_size > UINT_MAX - len)
+				return false;
 			bv->bv_len += len;
 			bio->bi_iter.bi_size += len;
 			return true;