selftests: tls: factor out cmsg send/receive

mainline inclusion
from mainline-v5.16-rc3
commit 31180adb
category: feature
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=31180adb0bed7d3e67ae97121dc83f320bbd45be



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Add helpers for sending and receiving special record types.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>