[L2TP]: Add the ability to autoload a pppox protocol module.

This patch allows a name "pppox-proto-nnn" to be used in modprobe.conf
to autoload a PPPoX protocol nnn.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>