Merge branch 'selftests-xsk-stabilize-timeout-test-behavior'

Tushar Vyavahare says:

====================
selftests/xsk: stabilize timeout test behavior

This series improves AF_XDP selftests by making timeout handling
explicit and fixing sources of non-determinism in xsk timeout tests.

Patch 1 introduces test_spec::poll_tmout and removes implicit
dependence on RX UMEM setup state for timeout behavior.

Patch 2 fixes thread harness sequencing by attaching XDP programs
before worker startup, removing signal-based termination, and using
barrier synchronization only for dual-thread runs.

Patch 3 restores shared_umem after POLL_TXQ_FULL so test-local
configuration does not leak into subsequent cases on shared-netdev
runs.

Together these changes make timeout handling easier to follow and
improve selftest stability, especially on real NIC runs.
====================

Link: https://patch.msgid.link/20260616154955.1492560-1-tushar.vyavahare@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>