ipv6: move ip6_dst_hoplimit() into core kernel

It will be used by vxlan, and may not be inlined.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>