selftests/bpf: Verify that rebinding to port < 1024 from BPF works

Return 3 to indicate that permission check for port 111
should be skipped.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210127193140.3170382-2-sdf@google.com