nvmet: avoid duplicate qid in connect cmd

According to the NVMe specification, if the host sends a Connect command
specifying a queue id which has already been created, a status value of
NVME_SC_CMD_SEQ_ERROR is returned.

Signed-off-by: Amit Engel <amit.engel@dell.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>