rdma: Allow demand loading of NETLINK_RDMA

Provide a module alias so that if userspace opens a netlink
socket for RDMA the kernel support is loaded automatically.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>