Restore loop break in vm_pageout_lowmem().

r355004 removed return statement from this loop with intention to also
call uma_reclaim_wakeup().  But in case of vm.lowmem_period=0 it causes
infinite loop.

Reviewed by:	markj
Sponsored by:	iXsystems, Inc.

sys/vm/vm_pageout.c

@@ -1974,6 +1974,7 @@ vm_pageout_lowmem(void)
 		 */
 		uma_reclaim(UMA_RECLAIM_TRIM);
 		ret = true;
+		break;
 	}
 
 	/*