netfilter: Remove unnecessary conversion to bool

Here we could use the '!=' expression to fix the following coccicheck
warning:

./net/netfilter/xt_nfacct.c:30:41-46: WARNING: conversion to bool not needed here

Reported-by: Tosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>