xfrm Fix use after free in __xfrm6_udp_encap_rcv.

A recent patch changed xfrm6_udp_encap_rcv to not
free the skb itself anymore but fogot the case
where xfrm4_udp_encap_rcv is called subsequently.

Fix this by moving the call to xfrm4_udp_encap_rcv
from __xfrm6_udp_encap_rcv to xfrm6_udp_encap_rcv.

Fixes: 221ddb72 ("xfrm: Support GRO for IPv6 ESP in UDP encapsulation")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>