selftests/bpf: Output the correct error code for pthread APIs

The return value of pthread API is the error code when the called
API fails, so output the return value instead of errno.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20230613080921.1623219-3-houtao@huaweicloud.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>