selftests/bpf: test_xsk: Fix __testapp_validate_traffic()'s return value

selftests/bpf: test_xsk: Fix __testapp_validate_traffic()'s return value

__testapp_validate_traffic is supposed to return an integer value that
tells if the test passed (0), failed (-1) or was skiped (2). It actually
returns a boolean in the end. This doesn't harm when the test is
successful but can lead to misinterpretation in case of failure as 1
will be returned instead of -1.

Return TEST_FAILURE (-1) in case of failure, TEST_PASS (0) otherwise.

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-v7-3-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>