vti/tunnel: Unify ikey/okey printing

vti/tunnel: Unify ikey/okey printing

For vti6 tunnel we print [io]key in dotted-quad notation
(ipv4 address) while in vti we do that in hex format.

For vti tunnel we print [io]key only if value is not
zero while for vti6 we miss such check.

Unify vti and vti6 tunnel [io]key output.

While here enlarge s2 buffer to the same size as in rest
of tunnel support code (64 bytes) and check return from
inet_ntop().

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>