ip/iptoken: fix dump error when ipv6 disabled

ip/iptoken: fix dump error when ipv6 disabled

When we disable IPv6 from the start up (ipv6.disable=1), there will be
no IPv6 route info in the dump message. If we return -1 when
ifi->ifi_family != AF_INET6, we will get error like

$ ip token list
Dump terminated

which will make user feel confused. There is no need to return -1 if the
dump message not match. Return 0 is enough.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>