[NETFILTER]: nfnetlink: Fix calculation of minimum message length

At least, valid nfnetlink message should have nlmsghdr and nfgenmsg.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>