RDMA/netlink: Convert LS to doit callback

RDMA_NL_LS protocol is actually does not dump anything,
but sets data and it should be handled by doit callback.

This patch actually converts RDMA_NL_LS to doit callback, while
preserving IWCM and RDMA_CM flows through netlink_dump_start().

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>