selftests/bpf: improve unexpected success reporting in test_syctl
When tests fail because sysctl() unexpectedly succeeds, they print an
inappropriate "Unexpected failure" message and a random errno. Zero
out errno before calling sysctl() and replace the message with
"Unexpected success".
Fixes: 1f5fa9ab6e2e ("selftests/bpf: Test BPF_CGROUP_SYSCTL") Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>