netfilter: use PTR_ERR_OR_ZERO()

Fix ptr_ret.cocci warnings:

  net/netfilter/xt_connlimit.c:96:1-3: WARNING: PTR_ERR_OR_ZERO can be used
  net/netfilter/nft_numgen.c:240:1-3: WARNING: PTR_ERR_OR_ZERO can be used

Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>