RDMA/netlink: Simplify code of autoload modules

The request_module() call is internally wrapped by CONFIG_MODULE,
so there is no need to check it in our RDMA code too.

Refactor to simplify the code.

Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>