netfilter: nf_nat: add nf_nat_csum()

Add a static function nf_nat_csum() to replace the duplicate code in
nf_nat_mangle_udp_packet() and __nf_nat_mangle_tcp_packet().

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