selftests/bpf: Add netlink helper library

Add a minimal netlink helper library for the BPF selftests. This has been
taken and cut down and cleaned up from iproute2. This covers basics such
as netdevice creation which we need for BPF selftests / BPF CI given
iproute2 package cannot cover it yet.

Stanislav Fomichev suggested that this could be replaced in future by ynl
tool generated C code once it has RTNL support to create devices. Once we
get to this point the BPF CI would also need to add libmnl. If no further
extensions are needed, a second option could be that we remove this code
again once iproute2 package has support.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20231024214904.29825-7-daniel@iogearbox.net
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>