xen: don't use sysret for sysexit32

When implementing sysexit32, don't let Xen use sysret to return to
userspace.  That results in usermode register state being trashed.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/xen/xen-asm_64.S

@@ -173,7 +173,7 @@ ENTRY(xen_sysexit)
 	pushq $__USER32_CS
 	pushq %rdx
 
-	pushq $VGCF_in_syscall
+	pushq $0
 1:	jmp hypercall_iret
 ENDPATCH(xen_sysexit)
 RELOC(xen_sysexit, 1b+1)