[PATCH] Fix block layer bug handling of partial bvec completion

Noted by Sean Estabrooks:

There is a bug in "ll_rw_blk.c" handling partial bvec submissions.
For whatever reason the floppy driver was triggering it more than
other users (most likely because most other devices tend to be able
to always complete a full request in one go).

Fixed by updating the proper bio vector index entry.

drivers/block/ll_rw_blk.c

@@ -2307,8 +2307,8 @@ static int __end_that_request_first(struct request *req, int uptodate,
 			 * not a complete bvec done
 			 */
 			if (unlikely(nbytes > nr_bytes)) {
-				bio_iovec(bio)->bv_offset += nr_bytes;
-				bio_iovec(bio)->bv_len -= nr_bytes;
+				bio_iovec_idx(bio, idx)->bv_offset += nr_bytes;
+				bio_iovec_idx(bio, idx)->bv_len -= nr_bytes;
 				bio_nbytes += nr_bytes;
 				total_bytes += nr_bytes;
 				break;