syncookies: make v4/v6 synflood warning behaviour the same

both syn_flood_warning functions print a message, but
ipv4 version only prints a warning if CONFIG_SYN_COOKIES=y.

Make the v4 one behave like the v6 one.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>