Make vm_page_sunbusy() assert that the page is unlocked.

Reviewed by:	kib
MFC after:	1 week
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D11946

sys/vm/vm_page.c

@@ -757,6 +757,7 @@ vm_page_sunbusy(vm_page_t m)
 {
 	u_int x;
 
+	vm_page_lock_assert(m, MA_NOTOWNED);
 	vm_page_assert_sbusied(m);
 
 	for (;;) {