nvme-rdma: fix error code in nvme_rdma_create_ctrl()

We accidentally return ERR_PTR(0) which is NULL.  The caller isn't
explicitly checking for that but I couldn't immediately spot whether
this would lead to a NULL dereference.  Anyway, we can fix add an
error code easily enough.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>