net/sched: flower: add missing validation of TCA_FLOWER_FLAGS

unlike other classifiers that can be offloaded (i.e. users can set flags
like 'skip_hw' and 'skip_sw'), 'cls_flower' doesn't validate the size of
netlink attribute 'TCA_FLOWER_FLAGS' provided by user: add a proper entry
to fl_policy.

Fixes: 5b33f488

 ("net/flower: Introduce hardware offload support")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>