s390/test_unwind: fix possible memleak in test_unwind()

test_unwind() misses to call kfree(bt) in an error path.
Add the missed function call to fix it.

Fixes: 06101546 ("s390/test_unwind: print verbose unwinding results")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/lib/test_unwind.c

@@ -64,6 +64,7 @@ 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;
 		}
 		sprint_symbol(sym, addr);