net: handle ARPHRD_IP6GRE in dev_is_mac_header_xmit()

Similar to commit 3b707c30 ("net: dev_is_mac_header_xmit() true for
ARPHRD_RAWIP"), add ARPHRD_IP6GRE to dev_is_mac_header_xmit(), to make
ip6gre compatible with act_mirred and __bpf_redirect().

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>