RDMA/core: Consider net ns of gid attribute for RoCE

When resolving destination address or route, when net namespace is
unavailable, refer to the net namespace of the netdevice of the SGID
attribute. This is typically the case for requests arriving from the
network for RoCE ports.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>