iproute2: ip6_tunnel mode bugfixes: any,vti6

iproute2: ip6_tunnel mode bugfixes: any,vti6

- any ipv6 tunnel mode (proto == 0) could not be set
due to incomplete set of cases in do_add, do_del.
- vti6 logic was inverted: it was using "ip6_vti0" basedev
UNLESS mode is set to vti6.

We don't need a switch by p.proto in do_add()/do_del(): it
already exists in parse_args(). So if parse_args() call
was successful, no need to check tunnel mode again.

Signed-off-by: Alexey Andriyanov <alan@al-an.info>