fix page_list_splice()

Other than in __list_splice(), the first element's prev pointer doesn't
need adjustment here - it already is PAGE_LIST_NULL. Rather than fixing
the assignment (to formally match __list_splice()), simply assert that
this assignment is really unnecessary.

Reported-by: Jisoo Yang <jisooy@xxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/include/xen/mm.h
+++ b/xen/include/xen/mm.h
@@ -270,7 +270,8 @@ page_list_splice(struct page_list_head *
     last = list->tail;
     at = head->next;
 
-    first->list.prev = page_to_pdx(head->next);
+    /* Both first->list.prev and at->list.prev are PAGE_LIST_NULL. */
+    ASSERT(first->list.prev == at->list.prev);
     head->next = first;
 
     last->list.next = page_to_pdx(at);