block: avoid blk_bio_segment_split for small I/O operations

void __blk_queue_split(struct request_queue *q, struct bio **bio,

		unsigned int *nr_segs)

{

	struct bio *split;

	struct bio *split = NULL;

	switch (bio_op(*bio)) {

	case REQ_OP_DISCARD:

				nr_segs);

		break;

	default:

		/*

		 * All drivers must accept single-segments bios that are <=

		 * PAGE_SIZE.  This is a quick and dirty check that relies on

		 * the fact that bi_io_vec[0] is always valid if a bio has data.

		 * The check might lead to occasional false negatives when bios

		 * are cloned, but compared to the performance impact of cloned

		 * bios themselves the loop below doesn't matter anyway.

		 */

		if (!q->limits.chunk_sectors &&

		    (*bio)->bi_vcnt == 1 &&

		    (*bio)->bi_io_vec[0].bv_len <= PAGE_SIZE) {

			*nr_segs = 1;

			break;

		}

		split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);

		break;

	}