nfp: bpf: refactor offload logic

We currently create a fake cls_bpf offload object when we want
to offload XDP.  Simplify and clarify the code by moving the
TC/XDP specific logic out of common offload code.  This is easy
now that we don't support legacy TC actions.  We only need the
bpf program and state of the skip_sw flag.

Temporarily set @code to NULL in nfp_net_bpf_offload(), compilers
seem to have trouble recognizing it's always initialized.  Next
patches will eliminate that variable.

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