nvmet: release a ns reference in nvmet_req_uninit if needed

nvmet_req_init looked up a namespace and took a reference on it (unless it
failed prior to that). If the request is uninitialized (in error cases) we
need to remove that reference in case it was taken, otherwise we leak
namespace reference when calling nvme_req_uninit.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>