proc: Put thread_pid in release_task not proc_flush_pid

Oleg pointed out that in the unlikely event the kernel is compiled
with CONFIG_PROC_FS unset that release_task will now leak the pid.

Move the put_pid out of proc_flush_pid into release_task to fix this
and to guarantee I don't make that mistake again.

When possible it makes sense to keep get and put in the same function
so it can easily been seen how they pair up.

Fixes: 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc")
Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

fs/proc/base.c

@@ -3274,7 +3274,6 @@ static const struct inode_operations proc_tgid_base_inode_operations = {
 void proc_flush_pid(struct pid *pid)
 {
 	proc_invalidate_siblings_dcache(&pid->inodes, &pid->lock);
-	put_pid(pid);
 }
 
 static struct dentry *proc_pid_instantiate(struct dentry * dentry,

kernel/exit.c

@@ -219,6 +219,7 @@ repeat:
 
 	write_unlock_irq(&tasklist_lock);
 	proc_flush_pid(thread_pid);
+	put_pid(thread_pid);
 	release_thread(p);
 	put_task_struct_rcu_user(p);