net: add CONFIG_NET_INGRESS to enable ingress filtering

This new config switch enables the ingress filtering infrastructure that is
controlled through the ingress_needed static key. This prepares the
introduction of the Netfilter ingress hook that resides under this unique
static key.

Note that CONFIG_SCH_INGRESS automatically selects this, that should be no
problem since this also depends on CONFIG_NET_CLS_ACT.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/Kconfig b/net/Kconfig
index 44dd578..57a7c5a 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -45,6 +45,9 @@
 	  Newly written code should NEVER need this option but do
 	  compat-independent messages instead!
 
+config NET_INGRESS
+	bool
+
 menu "Networking options"
 
 source "net/packet/Kconfig"