selftests: tls: clean up uninitialized warnings

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>