ip6gretap: Fix MTU to allow for Ethernet header

When we were creating an ip6gretap interface the MTU was about 6 bytes
short of what was needed.  It turns out we were not taking the Ethernet
header into account and as a result we were eating into the 8 bytes
reserved for the encap limit.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>