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>