When an I/O error occurs on page out, there is no need to dirty the page,

because it is already dirty.  Instead, assert that the page is dirty.

Reviewed by:	kib, markj
MFC after:	1 week

sys/vm/swap_pager.c

@@ -1524,7 +1524,7 @@ swp_pager_async_iodone(struct buf *bp)
 				 * so it doesn't clog the inactive list,
 				 * then finish the I/O.
 				 */
-				vm_page_dirty(m);
+				MPASS(m->dirty == VM_PAGE_BITS_ALL);
 				vm_page_lock(m);
 				vm_page_activate(m);
 				vm_page_unlock(m);