bpf, bpftool: enable recvmsg attach types

Trivial patch to bpftool in order to complete enabling attaching programs
to BPF_CGROUP_UDP{4,6}_RECVMSG.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>