bpf: offload: aggregate offloads per-device

Currently we have two lists of offloaded objects - programs and maps.
Netdevice unregister notifier scans those lists to orphan objects
associated with device being unregistered.  This puts unnecessary
(even if negligible) burden on all netdev unregister calls in BPF-
-enabled kernel.  The lists of objects may potentially get long
making the linear scan even more problematic.  There haven't been
complaints about this mechanisms so far, but it is suboptimal.

Instead of relying on notifiers, make the few BPF-capable drivers
register explicitly for BPF offloads.  The programs and maps will
now be collected per-device not on a global list, and only scanned
for removal when driver unregisters from BPF offloads.

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>