ipvs: use skb_to_full_sk() helper

SYNACK packets might be attached to request sockets.

Use skb_to_full_sk() helper to avoid illegal accesses to
inet_sk(skb->sk)

Fixes: ca6fb0651883 ("tcp: attach SYNACK messages to request sockets instead of listener")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
Acked-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>