s390/perf: obtain sie_block from the right address

Since commit 1179f170b6f0 ("s390: fix fpu restore in entry.S"), the
sie_block pointer is located at empty1[1], but in sie_block() it was
taken from empty1[0].

This leads to a random pointer being dereferenced, possibly causing
system crash.

This problem can be observed when running a simple guest with an endless
loop and recording the cpu-clock event:

sudo perf kvm --guestvmlinux=<guestkernel> --guest top -e cpu-clock

With this fix, the correct guest address is shown.

Fixes: 1179f170b6f0 ("s390: fix fpu restore in entry.S")
Cc: [e-mail]
Acked-by: Christian Borntraeger <[e-mail]>
Acked-by: Claudio Imbrenda <[e-mail]>
Reviewed-by: Heiko Carstens <[e-mail]>
Signed-off-by: Nico Boehr <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>

arch/s390/kernel/perf_event.c

···
 	if (!stack)
 		return NULL;
 
-	return (struct kvm_s390_sie_block *) stack->empty1[0];
+	return (struct kvm_s390_sie_block *)stack->empty1[1];
 }
 
 static bool is_in_guest(struct pt_regs *regs)