ip6_tunnel: process toobig in a better way

The same improvement in "ip6_gre: process toobig in a better way"
is needed by ip4ip6 and ip6ip6 as well.

Note that ip4ip6 and ip6ip6 will also update sk dst pmtu in their
err_handlers. Like I said before, gre6 could not do this as it's
inner proto is not certain. But for all of them, sk dst pmtu will
be updated in tx path if in need.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>