pid: fix lockdep deadlock warning due to ucount_lock

=========================================================
[ INFO: possible irq lock inversion dependency detected ]
4.10.0-rc2-00024-g4aecec9-dirty #118 Tainted: G W
---------------------------------------------------------
swapper/1/0 just changed the state of lock:
(&(&sighand->siglock)->rlock){-.....}, at: [<ffffffffbd0a1bc6>] __lock_task_sighand+0xb6/0x2c0
but this lock took another, HARDIRQ-unsafe lock in the past:
(ucounts_lock){+.+...}
and interrupts could create inverse lock ordering between them.
other info that might help us debug this:
Chain exists of: &(&sighand->siglock)->rlock --> &(&tty->ctrl_lock)->rlock --> ucounts_lock
Possible interrupt unsafe locking scenario:
CPU0 CPU1
---- ----
lock(ucounts_lock);
local_irq_disable();
lock(&(&sighand->siglock)->rlock);
lock(&(&tty->ctrl_lock)->rlock);
<Interrupt>
lock(&(&sighand->siglock)->rlock);

*** DEADLOCK ***

This patch removes a dependency between rlock and ucount_lock.

Fixes: f333c700c610 ("pidns: Add a limit on the number of pid namespaces")
Cc: [e-mail]
Signed-off-by: Andrei Vagin <[e-mail]>
Acked-by: Al Viro <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

kernel/pid_namespace.c

···
 
 static void delayed_free_pidns(struct rcu_head *p)
 {
-	kmem_cache_free(pid_ns_cachep,
-			container_of(p, struct pid_namespace, rcu));
+	struct pid_namespace *ns = container_of(p, struct pid_namespace, rcu);
+
+	dec_pid_namespaces(ns->ucounts);
+	put_user_ns(ns->user_ns);
+
+	kmem_cache_free(pid_ns_cachep, ns);
 }
 
 static void destroy_pid_namespace(struct pid_namespace *ns)
···
 	ns_free_inum(&ns->ns);
 	for (i = 0; i < PIDMAP_ENTRIES; i++)
 		kfree(ns->pidmap[i].page);
-	dec_pid_namespaces(ns->ucounts);
-	put_user_ns(ns->user_ns);
 	call_rcu(&ns->rcu, delayed_free_pidns);
 }