x86: fix HYPERVISOR_update_descriptor()

Impact: fix potential oops during app-initiated LDT manipulation

The underlying hypercall has differing argument requirements on 32-
and 64-bit.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
LKML-Reference: <49B9061E.76E4.0078.0@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/xen/hypercall.h

@@ -296,6 +296,8 @@ HYPERVISOR_get_debugreg(int reg)
 static inline int
 HYPERVISOR_update_descriptor(u64 ma, u64 desc)
 {
+	if (sizeof(u64) == sizeof(long))
+		return _hypercall2(int, update_descriptor, ma, desc);
 	return _hypercall4(int, update_descriptor, ma, ma>>32, desc, desc>>32);
 }