ipv6: make ipv6_anycast_destination logic usable without dst_entry
nft_fib_ipv6 uses ipv6_anycast_destination(), but upcoming patch removes
the dst_entry usage in favor of fib6_result.
Move the 'plen > 127' logic to a new helper and call it from the
existing one.
Signed-off-by: Florian Westphal <fw@strlen.de>