nvmet: remove extra variable in identify ns

We remove the extra local variable struct nvmet_ns in
nvmet_execute_identify_ns() since req already has ns member that can be
reused, this also eliminates the explicit call to nvmet_put_namespace()
which is already present in the request completion path.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>