CAN-Ethernet gateway - kernel part

CAN-Ethernet gateway - kernel part

This patch implements CAN-Ethernet (or more precisely CAN-UDP) gateway.
The code here is only responsible for routing the messages in kernel
space. Initial configuration and setup of the involved sockets is done
in user space with cegw utility.

Currently, the gateway support only IPv4 and non-CAN_FD packets, but the
implementation and user-space ABI should allow adding support for those
later.

Signed-off-by: Radek Matějka <radek.matejka@gmail.com>
Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>