nvme: don't call blk_mq_{,un}quiesce_tagset when ctrl->tagset is NULL

The NVMe drivers support a mode where no tagset is allocated for the I/O
queues and only the admin queue is usable.  In that case ctrl->tagset is
NULL and we must not call the block per-tagset quiesce helpers that
dereference it.

Fixes: 98d81f0d ("nvme: use blk_mq_[un]quiesce_tagset")
Reported-by: Gerd Bayer <gbayer@linux.ibm.com>
Reported-by: Chao Leng <lengchao@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Leng <lengchao@huawei.com>