nsproxy: add helper to go from arbitrary namespace to ns_common

They all contains struct ns_common ns and if there ever is one where
that isn't the case we'll catch it here at build time.
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/nsproxy.h

@@ -42,6 +42,17 @@ struct nsproxy {
 };
 extern struct nsproxy init_nsproxy;
 
+#define to_ns_common(__ns)                              \
+	_Generic((__ns),                                \
+		struct cgroup_namespace *: &(__ns->ns), \
+		struct ipc_namespace *:    &(__ns->ns), \
+		struct net *:              &(__ns->ns), \
+		struct pid_namespace *:    &(__ns->ns), \
+		struct mnt_namespace *:    &(__ns->ns), \
+		struct time_namespace *:   &(__ns->ns), \
+		struct user_namespace *:   &(__ns->ns), \
+		struct uts_namespace *:    &(__ns->ns))
+
 /*
  * A structure to encompass all bits needed to install
  * a partial or complete new set of namespaces.