staging: erofs: fix unexpected out-of-bound data access

Unexpected out-of-bound data will be read in erofs_read_raw_page
after commit 07173c3ec2 ("block: enable multipage bvecs") since
one iovec could have multiple pages.

Let's fix as what Ming's pointed out in the previous email [1].

[1] https://lore.kernel.org/lkml/20190411080953.GE421@ming.t460p/

Suggested-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Fixes: 07173c3ec2 ("block: enable multipage bvecs")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/erofs/data.c

@@ -298,7 +298,7 @@ submit_bio_retry:
 	*last_block = current_block;
 
 	/* shift in advance in case of it followed by too many gaps */
-	if (unlikely(bio->bi_vcnt >= bio->bi_max_vecs)) {
+	if (bio->bi_iter.bi_size >= bio->bi_max_vecs * PAGE_SIZE) {
 		/* err should reassign to 0 after submitting */
 		err = 0;
 		goto submit_bio_out;