Ensure that pass > 0 when starting a scan with vm_pages_needed == 1.

Otherwise the page daemon will not reclaim pages and thus will not
wake threads sleeping in VM_WAIT.

Reported and tested by:	pho
Reviewed by:	alc, kib
X-MFC with:	r327168
Differential Revision:	https://reviews.freebsd.org/D13640

sys/vm/vm_pageout.c

@@ -1836,6 +1836,8 @@ vm_pageout_worker(void *arg)
 			 */
 			if (vm_pages_needed) {
 				mtx_unlock(&vm_page_queue_free_mtx);
+				if (pass == 0)
+					pass++;
 			} else if (mtx_sleep(&vm_pageout_wanted,
 			    &vm_page_queue_free_mtx, PDROP | PVM, "psleep",
 			    hz) == 0) {