x86: constify local variables in mapcache_current_vcpu()

As of the referenced commit v won't be returned from the function anymore.
Yet that was the sole reason for it to not be pointer-to-const. curr,
which was introduced by that change, had no reason to be pointer-to-non-
const from the start.

Amends: 622c9a5ba9 ("x86/mm: accurately track which vCPU page-tables are loaded")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger@xenproject.org>

xen/arch/x86/domain_page.c

@@ -20,8 +20,8 @@
 
 static inline struct vcpu *mapcache_current_vcpu(void)
 {
-    struct vcpu *v = this_cpu(pgtable_vcpu);
+    const struct vcpu *v = this_cpu(pgtable_vcpu);
-    struct vcpu *curr = current;
+    const struct vcpu *curr = current;
 
     /*
      * During early boot pgtable_vcpu is not set, callers must handle NULL.