selftests/xsk: fix timeout thread harness sequencing

selftests/xsk: fix timeout thread harness sequencing

Prevent workers from running before XDP program attachment completes.
The previous ordering allowed races between worker startup and setup.

Attach XDP programs before entering traffic validation.

Remove SIGUSR1-based worker termination and use pthread_join() for
thread shutdown so blocking syscalls are not interrupted.

Use barriers only for dual-thread runs so participants match and
teardown ordering stays deterministic.

This removes setup/startup races and stabilizes harness sequencing.

Signed-off-by: Tushar Vyavahare <tushar.vyavahare@intel.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://patch.msgid.link/20260616154955.1492560-3-tushar.vyavahare@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>