ipv6: export fib6_lookup for nft_fib_ipv6

ipv6: export fib6_lookup for nft_fib_ipv6

Upcoming patch will call fib6_lookup from nft_fib_ipv6.  The EXPORT_SYMBOL is
added twice because there are two implementations of the function, one
is a small stub for MULTIPLE_TABLES=n, only one is compiled into the
kernel depending on .config settings.

Alternative to EXPORT_SYMBOL is to use an indirect call via the
ipv6_stub->fib6_lookup() indirection, but thats more expensive than the
direct call.

Also, nft_fib_ipv6 cannot be builtin if ipv6 is a module.

Signed-off-by: Florian Westphal <fw@strlen.de>