i386: fix unwind annotations in failsafe callback

This amends c/s 14245:b1f33c3a97fa (when the sparse Linux tree was
still part of the main Xen repo) in that it
- adds a missing annotation to the "pushl $-1"
- moves the RING0_INT_FRAME earlier (after adjusting the code to do the
  stack pointer adjustment before the conditional branch), thus
  increasing the covered region

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/arch/i386/kernel/entry-xen.S
+++ b/arch/i386/kernel/entry-xen.S
@@ -872,12 +872,11 @@ ENTRY(failsafe_callback)
 4:	mov 16(%esp),%gs
 	testl %eax,%eax
 	popl %eax
-	jz 5f
-	addl $16,%esp		# EAX != 0 => Category 2 (Bad IRET)
-	jmp iret_exc
-5:	addl $16,%esp		# EAX == 0 => Category 1 (Bad segment)
+	leal (%esp,16),%esp
 	RING0_INT_FRAME
-	pushl $-1
+	jnz iret_exc		# EAX != 0 => Category 2 (Bad IRET)
+	pushl $-1		# EAX == 0 => Category 1 (Bad segment)
+	CFI_ADJUST_CFA_OFFSET 4
 	SAVE_ALL
 	jmp ret_from_exception
 .section .fixup,"ax";		\