nfp: hand over to BPF offload app at coarser granularity

Instead of having an app callback per message type hand off
all offload-related handling to apps with one "rest of ndo_bpf"
callback.

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>