nvmet: set status to 0 in case for invalid nsid

For unallocated namespace in nvmet_execute_identify_ns() don't set the
status to NVME_SC_INVALID_NS, set it to zero.

Fixes: bffcd507 ("nvmet: set right status on error in id-ns handler")
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>