s390/unwind: avoid int overflow in outside_of_stack

When current task is interrupted in-between stack frame allocation
and backchain write instructions new stack frame backchain pointer
is left uninitialized. That invalid backchain value is passed into
outside_of_stack for sanity check. Make sure int overflow does not happen
by subtracting stack_frame size from the stack "end" rather than adding
it to "random" backchain value.

Fixes: 41b0474c1b1c ("s390/unwind: introduce stack unwind API")
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/unwind_bc.c

@@ -20,7 +20,7 @@ EXPORT_SYMBOL_GPL(unwind_get_return_address);
 static bool outside_of_stack(struct unwind_state *state, unsigned long sp)
 {
 	return (sp <= state->sp) ||
-		(sp + sizeof(struct stack_frame) > state->stack_info.end);
+		(sp > state->stack_info.end - sizeof(struct stack_frame));
 }
 
 static bool update_stack_info(struct unwind_state *state, unsigned long sp)