nvmet: remove duplicate NULL initialization for req->ns

Remove the duplicate NULL initialization for req->ns.  req->ns is always
initialized to NULL in nvmet_req_init(), so there is no need to reset
it later on failures unless we have previously assigned a value to it.

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