Remove unncessary atomic operation when reading process flags in

PMC_PROC_IS_USING_PMCS macro.

Invocations of this macro are not synchronized with setting/clearing
of P_HWPMC flag, so the atomic operation here isn't needed.  Removing
the atomic operation provides noticeable improvement (5-6%) on
some scheduler-intensive workloads with HWPMC_HOOKS enabled on an
8C Sandy Bridge Xeon system.

Sponsored by: Intel
Reviewed by: jhb
MFC after: 1 week

sys/sys/pmckern.h

@@ -201,8 +201,7 @@ do {						\
 
 /* Check if a process is using HWPMCs.*/
 #define PMC_PROC_IS_USING_PMCS(p)				\
-	(__predict_false(atomic_load_acq_int(&(p)->p_flag) &	\
-	    P_HWPMC))
+	(__predict_false(p->p_flag & P_HWPMC))
 
 /* Check if a thread have pending user capture. */
 #define PMC_IS_PENDING_CALLCHAIN(p)				\