selftests/bpf: test_xsk: Release resources when swap fails

selftests/bpf: test_xsk: Release resources when swap fails

testapp_validate_traffic() doesn't release the sockets and the umem
created by the threads if the test isn't currently in its last step.
Thus, if the swap_xsk_resources() fails before the last step, the
created resources aren't cleaned up.

Clean the sockets and the umem in case of swap_xsk_resources() 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-v7-7-39fe486593a3@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>