vm_page: add vm_page_clearref() helper

It is supposed to be used for ref_count manipulations when the pages
are owned by an object, but ref_count is used for something else than
the wiring, e.g. PTE population count on the page table page.

Reviewed by:	markj
Sponsored by:	Advanced Micro Devices (AMD)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D45910

sys/vm/vm_page.h

@@ -947,6 +947,17 @@ vm_page_in_laundry(vm_page_t m)
 	return (queue == PQ_LAUNDRY || queue == PQ_UNSWAPPABLE);
 }
 
+static inline void
+vm_page_clearref(vm_page_t m)
+{
+	u_int r;
+
+	r = m->ref_count;
+	while (atomic_fcmpset_int(&m->ref_count, &r, r & (VPRC_BLOCKED |
+	    VPRC_OBJREF)) == 0)
+		;
+}
+
 /*
  *	vm_page_drop:
  *