s390/test_unwind: unify error handling paths

Handle the case of "unwind state reliable but addr is 0" like other error
cases in this function and trigger output of failing stacktrace to aid
debugging.
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/lib/test_unwind.c

@@ -64,8 +64,8 @@ static noinline int test_unwind(struct task_struct *task, struct pt_regs *regs,
 			break;
 		if (state.reliable && !addr) {
 			pr_err("unwind state reliable but addr is 0\n");
-			kfree(bt);
-			return -EINVAL;
+			ret = -EINVAL;
+			break;
 		}
 		sprint_symbol(sym, addr);
 		if (bt_pos < BT_BUF_SIZE) {