rdma_rxe: Use netlink messages to add/delete links

Add support for the RDMA_NLDEV_CMD_NEWLINK/DELLINK messages which allow
dynamically adding new RXE links.  Deprecate the old module options for
now.

Cc: Moni Shoua <monis@mellanox.com>
Reviewed-by: Yanjun Zhu <yanjun.zhu@oracle.com>
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>