netdevsim: add support for simultaneous driver and hw XDP

Allow netdevsim to accept driver and offload attachment of XDP
BPF programs at the same time.

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