x86/shadow: unconditionally set the p2m/log-dirty allocation functions.

Otherwise enabling log-dirty mode on a PV guest that already has
a shadow allocation can leave the alloc/free functions pointers NULL,
and later try to dereference them.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>

xen/arch/x86/mm/shadow/common.c

@@ -3337,12 +3337,12 @@ static int shadow_one_bit_enable(struct domain *d, u32 mode)
             sh_set_allocation(d, 0, NULL);
             return -ENOMEM;
         }
-
-        /* Allow p2m and log-dirty code to borrow shadow memory */
-        d->arch.paging.alloc_page = shadow_alloc_p2m_page;
-        d->arch.paging.free_page = shadow_free_p2m_page;
     }
 
+    /* Allow p2m and log-dirty code to borrow shadow memory */
+    d->arch.paging.alloc_page = shadow_alloc_p2m_page;
+    d->arch.paging.free_page = shadow_free_p2m_page;
+
     if ( d->arch.paging.mode == 0 )
     {
         /* Init the shadow hash table */