netfilter: nf_tables_netdev: remove redundant ip_hdr assignment

We have already use skb_header_pointer to get the ip header pointer,
so there's no need to use ip_hdr again. Moreover, in NETDEV INGRESS
hook, ip header maybe not linear, so use ip_hdr is not appropriate,
remove it.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>