ip_tunnels: use tos and ttl fields also for IPv6

Rename the ipv4_tos and ipv4_ttl fields to just 'tos' and 'ttl', as they'll
be used with IPv6 tunnels, too.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>