initialise pi_lock if CONFIG_RT_MUTEXES=N

Fixes a bogus lockdep warning which causes lockdep to disable itself.
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

kernel/fork.c

@@ -933,8 +933,8 @@ asmlinkage long sys_set_tid_address(int __user *tidptr)
 
 static inline void rt_mutex_init_task(struct task_struct *p)
 {
-#ifdef CONFIG_RT_MUTEXES
 	spin_lock_init(&p->pi_lock);
+#ifdef CONFIG_RT_MUTEXES
 	plist_head_init(&p->pi_waiters, &p->pi_lock);
 	p->pi_blocked_on = NULL;
 #endif