x86/PV: make '0' debug key dump Dom0's stacks again

The conversion to __get_guest() failed to account for the fact that for
remote vCPU-s dumping gets done through a pointer obtained from
map_domain_page(): __get_guest() arranges for (apparent) accesses to
hypervisor space to cause #GP(0).

Fixes: 6a1d72d373 ('x86: split __{get,put}_user() into "guest" and "unsafe" variants')
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/traps.c

@@ -275,7 +275,9 @@ static void compat_show_guest_stack(struct vcpu *v,
     {
         if ( (((long)stack - 1) ^ ((long)(stack + 1) - 1)) & mask )
             break;
-        if ( __get_guest(addr, stack) )
+        if ( stack_page )
+            addr = *stack;
+        else if ( __get_guest(addr, stack) )
         {
             if ( i != 0 )
                 printk("\n    ");
@@ -344,7 +346,9 @@ static void show_guest_stack(struct vcpu *v, const struct cpu_user_regs *regs)
     {
         if ( (((long)stack - 1) ^ ((long)(stack + 1) - 1)) & mask )
             break;
-        if ( __get_guest(addr, stack) )
+        if ( stack_page )
+            addr = *stack;
+        else if ( __get_guest(addr, stack) )
         {
             if ( i != 0 )
                 printk("\n    ");