netdevsim: Set offsets to various protocol layers

The driver periodically generates "trapped" UDP packets that it then
passes on to devlink. Set the offsets to the various protocol layers.

This is a prerequisite to the next patch, where drop monitor is taught
to check that the offset to the MAC header was set.

Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>