arm/p2m: remove the page from p2m->pages list before freeing it

The p2m code is using the page list field to link all the pages used
for the stage-2 page tables. The page is added into the p2m->pages
list just after the allocation but never removed from the list.

The page list field is also used by the allocator, not removing may
result a later Xen crash due to inconsistency (see [1]).

This bug was introduced by the reworking of p2m code in commit 2ef3e36ec7
"xen/arm: p2m: Introduce p2m_set_entry and __p2m_set_entry".

[1] https://lists.xenproject.org/archives/html/xen-devel/2017-02/msg00524.html

Reported-by: Vijaya Kumar K <Vijaya.Kumar@cavium.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/arch/arm/p2m.c

@@ -660,6 +660,7 @@ static void p2m_free_entry(struct p2m_domain *p2m,
     unsigned int i;
     lpae_t *table;
     mfn_t mfn;
+    struct page_info *pg;
 
     /* Nothing to do if the entry is invalid. */
     if ( !p2m_valid(entry) )
@@ -697,7 +698,10 @@ static void p2m_free_entry(struct p2m_domain *p2m,
     mfn = _mfn(entry.p2m.base);
     ASSERT(mfn_valid(mfn));
 
-    free_domheap_page(mfn_to_page(mfn_x(mfn)));
+    pg = mfn_to_page(mfn_x(mfn));
+
+    page_list_del(pg, &p2m->pages);
+    free_domheap_page(pg);
 }
 
 static bool p2m_split_superpage(struct p2m_domain *p2m, lpae_t *entry,