block: add a missing BIOVEC_SEG_BOUNDARY check in bio_add_pc_page

The actual recaculation of segments in __blk_recalc_rq_segments will
do this check, so there is no point in forcing it if we know it won't
succeed.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -731,7 +731,9 @@ int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
 	}
 
 	/* If we may be able to merge these biovecs, force a recount */
-	if (bio->bi_vcnt > 1 && biovec_phys_mergeable(bvec-1, bvec))
+	if (bio->bi_vcnt > 1 &&
+	    biovec_phys_mergeable(bvec - 1, bvec) &&
+	    BIOVEC_SEG_BOUNDARY(q, bvec - 1, bvec))
 		bio_clear_flag(bio, BIO_SEG_VALID);
 
  done: