selftests: xsk: Return correct error codes

Return the correct error codes so they can be printed correctly.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-5-magnus.karlsson@gmail.com