selftests: tls: avoid flakiness in data_steal

selftests: tls: avoid flakiness in data_steal

We see the following failure a few times a week:

  #  RUN           global.data_steal ...
  # tls.c:3280:data_steal:Expected recv(cfd, buf2, sizeof(buf2), MSG_DONTWAIT) (10000) == -1 (-1)
  # data_steal: Test failed
  #          FAIL  global.data_steal
  not ok 8 global.data_steal

The 10000 bytes read suggests that the child process did a recv()
of half of the data using the TLS ULP and we're now getting the
remaining half. The intent of the test is to get the child to
enter _TCP_ recvmsg handler, so it needs to enter the syscall before
parent installed the TLS recvmsg with setsockopt(SOL_TLS).

Instead of the 10msec sleep send 1 byte of data and wait for the
child to consume it.

Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20260106200205.1593915-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>