selftests/bpf: test_xsk: Don't exit immediately when workers fail

selftests/bpf: test_xsk: Don't exit immediately when workers fail

TX and RX workers can fail in many places. These failures trigger a call
to exit_with_error() which exits the program immediately. It prevents the
following tests from running and isn't compliant with the CI.

Add return value to functions that can fail.
Handle failures more smoothly through report_failure().

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Link: https://lore.kernel.org/r/20251031-xsk-v7-11-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>