[IPV6]: Unify ip6_onlink() and ipip6_onlink().

Both are identical, let's create ipv6_chk_prefix() and use it
in both places.

diff --cc include/net/addrconf.h
Simple merge

diff --cc net/ipv6/addrconf.c
Simple merge

diff --cc net/ipv6/anycast.c
Simple merge

diff --cc net/ipv6/sit.c
Simple merge