blk-merge: fix blk_recount_segments

QUEUE_FLAG_NO_SG_MERGE is set at default for blk-mq devices,
so bio->bi_phys_segment computed may be bigger than
queue_max_segments(q) for blk-mq devices, then drivers will
fail to handle the case, for example, BUG_ON() in
virtio_queue_rq() can be triggerd for virtio-blk:

	https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1359146



This patch fixes the issue by ignoring the QUEUE_FLAG_NO_SG_MERGE
flag if the computed bio->bi_phys_segment is bigger than
queue_max_segments(q), and the regression is caused by commit
05f1dd53(block: add queue flag for disabling SG merging).

Reported-by: Kick In <pierre-andre.morey@canonical.com>
Tested-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>