pid_namespace: make freeing struct pid_namespace rcu-delayed

makes procfs ->premission() instances safety in RCU mode independent
from vfsmount_lock.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index e277266..7246ef3 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -23,6 +23,7 @@
 struct pid_namespace {
 	struct kref kref;
 	struct pidmap pidmap[PIDMAP_ENTRIES];
+	struct rcu_head rcu;
 	int last_pid;
 	unsigned int nr_hashed;
 	struct task_struct *child_reaper;