compat/memory: fix build with old gcc

struct xen_add_to_physmap_batch's size field being uint16_t causes old
compiler versions to warn about the pointless range check done inside
compat_handle_okay().

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Tested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

--- a/xen/common/compat/memory.c
+++ b/xen/common/compat/memory.c
@@ -206,18 +206,20 @@ int compat_memory_op(unsigned int cmd, X
         {
             unsigned int limit = (COMPAT_ARG_XLAT_SIZE - sizeof(*nat.atpb))
                                  / (sizeof(nat.atpb->idxs.p) + sizeof(nat.atpb->gpfns.p));
+            /* Use an intermediate variable to suppress warnings on old gcc: */
+            unsigned int size = cmp.atpb.size;
             xen_ulong_t *idxs = (void *)(nat.atpb + 1);
             xen_pfn_t *gpfns = (void *)(idxs + limit);
 
             if ( copy_from_guest(&cmp.atpb, compat, 1) ||
-                 !compat_handle_okay(cmp.atpb.idxs, cmp.atpb.size) ||
-                 !compat_handle_okay(cmp.atpb.gpfns, cmp.atpb.size) ||
-                 !compat_handle_okay(cmp.atpb.errs, cmp.atpb.size) )
+                 !compat_handle_okay(cmp.atpb.idxs, size) ||
+                 !compat_handle_okay(cmp.atpb.gpfns, size) ||
+                 !compat_handle_okay(cmp.atpb.errs, size) )
                 return -EFAULT;
 
             end_extent = start_extent + limit;
-            if ( end_extent > cmp.atpb.size )
-                end_extent = cmp.atpb.size;
+            if ( end_extent > size )
+                end_extent = size;
 
             idxs -= start_extent;
             gpfns -= start_extent;