net: Pre-create nl diag sk

The setns() syscall (called by switch_ns()) can be extremely
slow. If we call it two or more times from the same task the
kernel will synchonously go on a very slow routine called
synchronize_rcu() trying to put a reference on old namespaces.

To avoid doing this more than once I propose to create all
per-ns sockets in one place with one setns call. In this
patch there's on nl diag socket used to collect other sockets
is created this way.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/namespaces.h

@@ -21,6 +21,11 @@ struct ns_id {
 			struct mount_info *mntinfo_list;
 			struct mount_info *mntinfo_tree;
 		} mnt;
+
+		struct {
+			int nlsk;	/* for sockets collection */
+			int seqsk;	/* to talk to parasite daemons */
+		} net;
 	};
 };
 extern struct ns_id *ns_ids;