x86/xen: fix SLS mitigation in xen_hypercall_iret()

The backport of upstream patch a2796dff ("x86/xen: don't do PV iret
hypercall through hypercall page") missed to adapt the SLS mitigation
config check from CONFIG_MITIGATION_SLS to CONFIG_SLS.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/xen/xen-asm.S

@@ -221,7 +221,7 @@ SYM_CODE_END(xen_early_idt_handler_array)
 	push %rax
 	mov  $__HYPERVISOR_iret, %eax
 	syscall		/* Do the IRET. */
-#ifdef CONFIG_MITIGATION_SLS
+#ifdef CONFIG_SLS
 	int3
 #endif
 .endm