blk_hw_contig_segment(): bad segment size checks

Two bugs in there:

- The virt oversize check should use the current bio hardware back
  size and the next bio front size, not the same bio. Spotted by
  Neil Brown.

- The segment size check should add hw front sizes, not total bio
  sizes. Spotted by James Bottomley

Acked-by: James Bottomley <James.Bottomley@SteelEye.com>
Acked-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/ll_rw_blk.c

@@ -1304,9 +1304,9 @@ static int blk_hw_contig_segment(request_queue_t *q, struct bio *bio,
 	if (unlikely(!bio_flagged(nxt, BIO_SEG_VALID)))
 		blk_recount_segments(q, nxt);
 	if (!BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)) ||
-	    BIOVEC_VIRT_OVERSIZE(bio->bi_hw_front_size + bio->bi_hw_back_size))
+	    BIOVEC_VIRT_OVERSIZE(bio->bi_hw_back_size + nxt->bi_hw_front_size))
 		return 0;
-	if (bio->bi_size + nxt->bi_size > q->max_segment_size)
+	if (bio->bi_hw_back_size + nxt->bi_hw_front_size > q->max_segment_size)
 		return 0;
 
 	return 1;