netfilter: nftables: add nft_pernet() helper function

Consolidate call to net_generic(net, nf_tables_net_id) in this
wrapper function.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Author: ummakynes

include/net/netfilter/nf_tables.h

@@ -13,6 +13,7 @@
 #include <net/netfilter/nf_flow_table.h>
 #include <net/netlink.h>
 #include <net/flow_offload.h>
+#include <net/netns/generic.h>
 
 #define NFT_MAX_HOOKS	(NF_INET_INGRESS + 1)
 
@@ -1580,4 +1581,11 @@ struct nftables_pernet {
 	u8			validate_state;
 };
 
+extern unsigned int nf_tables_net_id;
+
+static inline struct nftables_pernet *nft_pernet(const struct net *net)
+{
+	return net_generic(net, nf_tables_net_id);
+}
+
 #endif /* _NET_NF_TABLES_H */