selftests: tls: clean up uninitialized warnings

mainline inclusion
from mainline-v5.14-rc1
commit baa00119
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I56S5K?from=project-issue

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=baa00119d69e3318da8d99867fc1170ebddf09ce



--------------------------------

A bunch of tests uses uninitialized stack memory as random
data to send. This is harmless but generates compiler warnings.
Explicitly init the buffers with random data.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Vadim Fedorenko <vfedorenko@novek.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>