x86/PV: conditionalize arch_set_info_guest()'s call to update_cr3()

sh_update_paging_modes() as its last action already invokes
sh_update_cr3(). Therefore there is no reason to invoke update_cr3()
another time immediately after calling paging_update_paging_modes(),
especially as sh_update_cr3() does not short-circuit the "nothing
changed" case.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/domain.c

@@ -1445,8 +1445,8 @@ int arch_set_info_guest(
 
     if ( paging_mode_enabled(d) )
         paging_update_paging_modes(v);
-
-    update_cr3(v);
+    else
+        update_cr3(v);
 #endif /* CONFIG_PV */
 
  out: