ipv6: Constify the sk parameter of several helper functions.

icmpv6_flow_init(), ip6_datagram_flow_key_init() and ip6_mc_hdr() don't
need to modify their sk argument. Make that explicit using const.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>