net/sched: matchall: add missing validation of TCA_MATCHALL_FLAGS

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

Fixes: b87f7936 ("net/sched: Add match-all classifier hw offloading.")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>