x86/mm: Fix p2m teardown locking

Holding the p2m lock during a p2m teardown, while unsharing entries pointing to
shared frames, causes a locking inversion and deadlock panic.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>

xen/arch/x86/mm/p2m.c

@@ -343,8 +343,6 @@ void p2m_teardown(struct p2m_domain *p2m)
     if (p2m == NULL)
         return;
 
-    p2m_lock(p2m);
-
 #ifdef __x86_64__
     for ( gfn=0; gfn < p2m->max_mapped_pfn; gfn++ )
     {
@@ -358,6 +356,8 @@ void p2m_teardown(struct p2m_domain *p2m)
     }
 #endif
 
+    p2m_lock(p2m);
+
     p2m->phys_table = pagetable_null();
 
     while ( (pg = page_list_remove_head(&p2m->pages)) )