ipv6: export ip6_datagram_recv_ctl

ip6_datagram_recv_ctl and ip6_datagram_send_ctl are used for handling IPv6
ancillary data.  Since ip6_datagram_send_ctl is already publicly exported for
use in modules, ip6_datagram_recv_ctl should also be available to support
ancillary data in the receive path.

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>