blk-merge: recaculate segment if it isn't less than max segments

The problem is introduced by commit 764f612c(blk-merge:
don't compute bi_phys_segments from bi_vcnt for cloned bio),
and merge is needed if number of current segment isn't less than
max segments.

Strictly speaking, bio->bi_vcnt shouldn't be used here since
it may not be accurate in cases of both cloned bio or bio cloned
from, but bio_segments() is a bit expensive, and bi_vcnt is still
the biggest number, so the approach should work.
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-merge.c

@@ -99,16 +99,17 @@ void blk_recount_segments(struct request_queue *q, struct bio *bio)
 {
 	bool no_sg_merge = !!test_bit(QUEUE_FLAG_NO_SG_MERGE,
 			&q->queue_flags);
+	bool merge_not_need = bio->bi_vcnt < queue_max_segments(q);
 
 	if (no_sg_merge && !bio_flagged(bio, BIO_CLONED) &&
-			bio->bi_vcnt < queue_max_segments(q))
+			merge_not_need)
 		bio->bi_phys_segments = bio->bi_vcnt;
 	else {
 		struct bio *nxt = bio->bi_next;
 
 		bio->bi_next = NULL;
 		bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio,
-				no_sg_merge);
+				no_sg_merge && merge_not_need);
 		bio->bi_next = nxt;
 	}