IB/core: Register SA ibnl client during ib_core initialization

Move SA ibnl client registration to ib_core module init.
This will allow us to register a single client to handle
all RDMA_NL_LS operations and make it SA independent.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>