netfilter: ipt_REJECT: postpone the checksum calculation.

postpone the checksum calculation, then if the output NIC supports checksum
offloading, we can utlize it. And though the output NIC doesn't support
checksum offloading, but we'll mangle this packet, this can free us from
updating the checksum, as the checksum calculation occurs later.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>