gnttab: fix "don't use possibly unbounded tail calls"

The compat mode code also needs adjustment to deal with the changed
return value from gnttab_copy().

This is part of XSA-226.

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

xen/common/compat/grant_table.c

@@ -258,9 +258,9 @@ int compat_grant_table_op(unsigned int cmd,
                 rc = gnttab_copy(guest_handle_cast(nat.uop, gnttab_copy_t), n);
             if ( rc > 0 )
             {
-                ASSERT(rc < n);
-                i -= n - rc;
-                n = rc;
+                ASSERT(rc <= n);
+                i -= rc;
+                n -= rc;
             }
             if ( rc >= 0 )
             {