selftests/bpf: add offload test based on netdevsim

Add a test of BPF offload control path interfaces based on
just-added netdevsim driver.  Perform various checks of both
the stack and the expected driver behaviour.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>