RDMA/cma: Avoid GID lookups on iWARP devices

We would like to enable the use of siw on top of a VPN that is
constructed and managed via a tun device. That hasn't worked up
until now because ARPHRD_NONE devices (such as tun devices) have
no GID for the RDMA/core to look up.

But it turns out that the egress device has already been picked for
us -- no GID is necessary. addr_handler() just has to do the right
thing with it.

Link: https://lore.kernel.org/r/168960675257.3007.4737911174148394395.stgit@manet.1015granger.net
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>