ip6_gre: set DSCP for non-IP

The current code always forces a dscp of 0 for all non-IP frames.
However when setting a specific TOS with the command

ip link add name tep0 type ip6gretap local fdd1:ced0:5d88:3fce::1
remote fdd1:ced0:5d88:3fce::2 tos 0xa0

one would expect all GRE encapsulated frames to have a TOS of 0xA0.
and not only when the payload is IPv4/IPv6.

Signed-off-by: Matthias May <matthias.may@westermo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>