bpf: offload: report device information about offloaded maps

Tell user space about device on which the map was created.
Unfortunate reality of user ABI makes sharing this code
with program offload difficult but the information is the
same.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>