bio: fix memcpy corruption in bio_copy_user_iov()

st driver uses blk_rq_map_user() in order to just build a request out
of page frames. In this case, map_data->offset is a non zero value and
iov[0].iov_base is NULL. We need to increase nr_pages for that.

Cc: stable@kernel.org
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

fs/bio.c

@@ -817,6 +817,9 @@ struct bio *bio_copy_user_iov(struct request_queue *q,
 		len += iov[i].iov_len;
 	}
 
+	if (offset)
+		nr_pages++;
+
 	bmd = bio_alloc_map_data(nr_pages, iov_count, gfp_mask);
 	if (!bmd)
 		return ERR_PTR(-ENOMEM);