nvmet: return uniform error for invalid ns

For nvmet_find_namespace() error case we have inconsistent error code
mapping in the function nvmet_get_smart_log_nsid() and
nvmet_set_feat_write_protect().

There is no point in retrying for the invalid namesapce from the host
side. Set the error code to the NVME_SC_INVALID_NS | NVME_SC_DNR which
matches what we have in nvmet_execute_identify_desclist().

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