net: sched: add extack support for offload via tc_cls_common_offload

Add extack support for hardware offload of classifiers. In order
to achieve this, a pointer to a struct netlink_ext_ack is added to the
struct tc_cls_common_offload that is passed to the callback for setting
up the classifier. Function tc_cls_common_offload_init() is updated to
support initialization of this new attribute.

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