selftests/xsk: restore shared_umem after POLL_TXQ_FULL

selftests/xsk: restore shared_umem after POLL_TXQ_FULL

POLL_TXQ_FULL temporarily disables shared_umem on TX to exercise the
TX timeout path in isolation.

With shared_umem enabled, TX setup expects RX UMEM to be initialized
first and fails with: "RX UMEM is not initialized before shared-UMEM TX
setup".

Save and restore shared_umem around POLL_TXQ_FULL execution, and restore
it on both success and pkt_stream_replace() failure paths.

Also add an in-code comment explaining why shared_umem is temporarily
disabled in this test.

This keeps timeout setup local and prevents cross-test state leakage.

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/20260616154955.1492560-4-tushar.vyavahare@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>