xen/gnttab: address a violation of MISRA C Rule 13.6

guest_handle_ok()'s expansion contains a sizeof() involving its
first argument guest_handle_cast().
The expansion of the latter, in turn, contains a variable
initialization.

Since MISRA considers the initialization (even of a local variable)
a side effect, the chain of expansions mentioned above violates
MISRA C:2012 Rule 13.6 (The operand of the `sizeof' operator shall not
contain any expression which has potential side effect).

Refactor the code to address the rule violation.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Federico Serafini <federico.serafini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/common/compat/grant_table.c

@@ -78,12 +78,15 @@ int compat_grant_table_op(
         cmd_op = cmd;
     switch ( cmd_op )
     {
-#define CASE(name) \
-    case GNTTABOP_##name: \
-        if ( unlikely(!guest_handle_okay(guest_handle_cast(uop, \
-                                                           gnttab_##name##_compat_t), \
-                                         count)) ) \
-            rc = -EFAULT; \
+#define CASE(name)                                                  \
+    case GNTTABOP_ ## name:                                         \
+    {                                                               \
+        XEN_GUEST_HANDLE_PARAM(gnttab_ ## name ## _compat_t) h =    \
+            guest_handle_cast(uop, gnttab_ ## name ## _compat_t);   \
+                                                                    \
+        if ( unlikely(!guest_handle_okay(h, count)) )               \
+            rc = -EFAULT;                                           \
+    }                                                               \
         break
 
 #ifndef CHECK_gnttab_map_grant_ref