udp: unbreak build lacking CONFIG_XFRM

We must use pre-processor conditional block or suitable accessors to
manipulate skb->sp elsewhere builds lacking the CONFIG_XFRM will break.

Fixes: dce4551c ("udp: preserve head state for IP_CMSG_PASSSEC")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>