net: cls_bpf: limit hardware offload by software-only flag

Add cls_bpf support for the TCA_CLS_FLAGS_SKIP_HW flag.
Unlike U32 and flower cls_bpf already has some netlink
flags defined.  Create a new attribute to be able to use
the same flag values as the above.

Unlike U32 and flower reject unknown flags.

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