netfilter: nat: nf_nat_mangle_{udp,tcp}_packet returns boolean

nf_nat_mangle_{udp,tcp}_packet() returns int. However, it is used as
bool type in many spots. Fix this by consistently handle this return
value as a boolean.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>