x86/domctl: Fix unitialized copyback in XEN_DOMCTL_PSR_GET_*

domctl_psr_get_val() copies unitialized stack space back if psr_get_val
fails. Fix by zero-initializing v_.

Fixes: 03f30dc193 ("x86: refactor psr: L3 CAT: implement get value flow.")
Signed-off-by: Johann Höpfner <hoepf@cit.tum.de>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/domctl.c

@@ -1347,7 +1347,7 @@ long arch_do_domctl(
             break;
 
 #define domctl_psr_get_val(d, domctl, type, copyback) ({    \
-    uint32_t v_;                                            \
+    uint32_t v_ = 0;                                        \
     int r_ = psr_get_val((d), (domctl)->u.psr_alloc.target, \
                          &v_, (type));                      \
                                                             \