netfilter: synproxy: fix sparse errors

Fix verbose sparse errors :

make C=2 CF=-D__CHECK_ENDIAN__ net/ipv4/netfilter/ipt_SYNPROXY.o

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>