locking/pv-qspinlock: Use cmpxchg_release() in __pv_queued_spin_unlock()

cmpxchg_release() is more lighweight than cmpxchg() on some archs(e.g.
PPC), moreover, in __pv_queued_spin_unlock() we only needs a RELEASE in
the fast path(pairing with *_try_lock() or *_lock()). And the slow path
has smp_store_release too. So it's safe to use cmpxchg_release here.

Suggested-by:  Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: benh@kernel.crashing.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: mpe@ellerman.id.au
Cc: paulmck@linux.vnet.ibm.com
Cc: paulus@samba.org
Cc: virtualization@lists.linux-foundation.org
Cc: waiman.long@hpe.com
Link: http://lkml.kernel.org/r/1474277037-15200-2-git-send-email-xinhui.pan@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/qspinlock_paravirt.h

@@ -540,7 +540,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
 	 * unhash. Otherwise it would be possible to have multiple @lock
 	 * entries, which would be BAD.
 	 */
-	locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
+	locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0);
 	if (likely(locked == _Q_LOCKED_VAL))
 		return;