selftests/xsk: make poll timeout mode explicit

selftests/xsk: make poll timeout mode explicit

Stop inferring timeout behavior from RX UMEM initialization state.
That ties timeout semantics to setup internals and obscures intent.

Use test_spec::poll_tmout as the explicit timeout-mode selector in
TX and RX paths.

In RX, treat poll timeout as expected only in timeout mode.
In TX, let send_pkts() own loop completion in non-timeout mode
and use __send_pkts() only for progress and timeout detection.

This makes timeout logic explicit and keeps control flow predictable.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
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/20260616154955.1492560-2-tushar.vyavahare@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>