nvmet: remove extra variable in id-ns handler

In nvmet_execute_identify_ns() local variable ctrl is accessed only in
one place, remove that and directly use it from nvmet_req->sq->ctrl.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>