[RFC] Fix failure path in alloc_pid()

[Matthew Wilcox]

The failure path removes the allocated PIDs from the wrong namespace.
I believe this is correct, but have not tested it.  Spotted by inspection,
do we have a test suite for PID namespaces?  Some error injection,
perhaps?

Fixes: 95846ecf9dac ("pid: replace pid bitmap implementation with IDR API")

diff --git a/kernel/pid.c b/kernel/pid.c
index b2f6c506035da..75264e0d1e71d 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -233,8 +233,11 @@ struct pid *alloc_pid(struct pid_namespace *ns)
 
 out_free:
        spin_lock_irq(&pidmap_lock);
-       while (++i <= ns->level)
-               idr_remove(&ns->idr, (pid->numbers + i)->nr);
+       upid = pid->numbers + i;
+       while (++i <= ns->level) {
+               upid++;
+               idr_remove(&upid->ns->idr, upid->nr);
+       }
 
        /* On failure to allocate the first pid, reset the state */
        if (ns->pid_allocated == PIDNS_ADDING)