net/ipv6: Rename rt6_multipath_select

Rename rt6_multipath_select to fib6_multipath_select and export it.
A later patch wants access to it similar to IPv4's fib_select_path.

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>