x86/shadow: fix PAE check for top-level table unshadowing

Clearly within the for_each_vcpu() the vCPU of this loop is meant, not
the (loop invariant) one the fault occurred on.

Fixes: 3d5e6a3ff3 ("x86 hvm: implement HVMOP_pagetable_dying")
Fixes: ef3b0d8d2c ("x86/shadow: shadow_table[] needs only one entry for PV-only configs")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

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

@@ -2677,10 +2677,10 @@ static int cf_check sh_page_fault(
 #if GUEST_PAGING_LEVELS == 3
             unsigned int i;
 
-            for_each_shadow_table(v, i)
+            for_each_shadow_table(tmp, i)
             {
                 mfn_t smfn = pagetable_get_mfn(
-                                 v->arch.paging.shadow.shadow_table[i]);
+                                 tmp->arch.paging.shadow.shadow_table[i]);
 
                 if ( mfn_x(smfn) )
                 {