genetlink: add auto module loading

When testing L2TP support, I discovered that the l2tp module is not autoloaded
as are other netlink interfaces. There is because of lack of hook in genetlink to call
request_module and load the module.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>