Use flsl() here rather than ffsl()

I discovered that we were computing page_order differently than linux.

MFC after:	3 days

sys/dev/drm/drm_bufs.c

@@ -1106,7 +1106,7 @@ int drm_order(unsigned long size)
 	if (size == 0)
 		return 0;
 
-	order = ffsl(size) - 1;
+	order = flsl(size) - 1;
 	if (size & ~(1ul << order))
 		++order;