sched: cls_flow: use skb_to_full_sk() helper

SYNACK packets might be attached to request sockets.

Fixes: ca6fb065

 ("tcp: attach SYNACK messages to request sockets instead of listener")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>