block: set .bi_max_vecs as actual allocated vector number

bvec_alloc() may allocate more bio vectors than requested, so set
.bi_max_vecs as actual allocated vector number, instead of the requested
number. This way can help fs build bigger bio because new bio often won't
be allocated until the current one becomes full.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -505,12 +505,13 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, unsigned int nr_iovecs,
 			goto err_free;
 
 		bio->bi_flags |= idx << BVEC_POOL_OFFSET;
+		bio->bi_max_vecs = bvec_nr_vecs(idx);
 	} else if (nr_iovecs) {
 		bvl = bio->bi_inline_vecs;
+		bio->bi_max_vecs = inline_vecs;
 	}
 
 	bio->bi_pool = bs;
-	bio->bi_max_vecs = nr_iovecs;
 	bio->bi_io_vec = bvl;
 	return bio;