locking/pvqspinlock: Replace xchg() by the more descriptive set_mb()

The xchg() function was used in pv_wait_node() to set a certain
value and provide a memory barrier which is what the set_mb()
function is for. This patch replaces the xchg() call by
set_mb().

Suggested-by: Linus Torvalds <[e-mail]>
Signed-off-by: Waiman Long <[e-mail]>
Cc: Douglas Hatch <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Scott J Norton <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/locking/qspinlock_paravirt.h

···
 		 *
 		 * Matches the xchg() from pv_kick_node().
 		 */
-		(void)xchg(&pn->state, vcpu_halted);
+		set_mb(pn->state, vcpu_halted);
 
 		if (!READ_ONCE(node->locked))
 			pv_wait(&pn->state, vcpu_halted);