locking/pvqspinlock: Fix a bug in qstat_read()

It's obviously wrong to set stat to NULL. So lets remove it.
Otherwise it is always zero when we check the latency of kick/wake.

Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Waiman Long <Waiman.Long@hpe.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1468405414-3700-1-git-send-email-xinhui.pan@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>