ip_tunnels: use u8/u16/u32

The ip_tunnels.h include file uses mixture of __u16 and u16 (etc.) types.
Unify it to the non-underscore variants.

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