nvmet: remove extra variable in id-desclist

We remove the extra local variable struct nvmet_ns in
nvmet_execute_identify_desclist() 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>