ipv6: Fix the upper MTU limit in GRE tunnel

Unlike ipv4, the struct member hlen holds the length of the GRE and ipv6
headers. This length is also counted in dev->hard_header_len.
Perhaps, it's more clean to modify the hlen to count only the GRE header
without ipv6 header as the variable name suggest, but the simple way to fix
this without regression risk is simply modify the calculation of the limit
in ip6gre_tunnel_change_mtu function.
Verified in kernel version v3.11.

Signed-off-by: Oussama Ghorbel <ou.ghorbel@gmail.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>