swap_pager_find_least(): assert that the function is called on the right object type

Reviewed by:	markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D37024

sys/vm/swap_pager.c

@@ -2259,6 +2259,7 @@ swap_pager_find_least(vm_object_t object, vm_pindex_t pindex)
 	int i;
 
 	VM_OBJECT_ASSERT_LOCKED(object);
+	MPASS((object->flags & OBJ_SWAP) != 0);
 
 	if (pctrie_is_empty(&object->un_pager.swp.swp_blks))
 		return (object->size);