Add basic RDMA netlink infrastructure that allows for registration of
RDMA clients for which data is to be exported and supplies message
construction callbacks.
Signed-off-by: NNir Muchtar <nirm@voltaire.com>

[ Reorganize a few things, add CONFIG_NET dependency.  - Roland ]
Signed-off-by: NRoland Dreier <roland@purestorage.com>