[UDP]: Cleanup UDP encapsulation code

This cleanup fell out after adding L2TP support where a new encap_rcv
funcptr was added to struct udp_sock. Have XFRM use the new encap_rcv
funcptr, which allows us to move the XFRM encap code from udp.c into
xfrm4_input.c.

Make xfrm4_rcv_encap() static since it is no longer called externally.

Signed-off-by: James Chapman <jchapman@katalix.com>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>