nfp: move basic eBPF stats to app-specific code

Allow apps to associate private data with vNICs and move
BPF-specific fields of nfp_net to such structure.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>