net: make default_rps_mask a per netns attribute

That really was meant to be a per netns attribute from the beginning.

The idea is that once proper isolation is in place in the main
namespace, additional demux in the child namespaces will be redundant.
Let's make child netns default rps mask empty by default.

To avoid bloating the netns with a possibly large cpumask, allocate
it on-demand during the first write operation.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/core.h

@@ -6,6 +6,7 @@
 
 struct ctl_table_header;
 struct prot_inuse;
+struct cpumask;
 
 struct netns_core {
 	/* core sysctls */
@@ -17,6 +18,10 @@ struct netns_core {
 #ifdef CONFIG_PROC_FS
 	struct prot_inuse __percpu *prot_inuse;
 #endif
+
+#if IS_ENABLED(CONFIG_RPS) && IS_ENABLED(CONFIG_SYSCTL)
+	struct cpumask *rps_default_mask;
+#endif
 };
 
 #endif