net/sched: act_pedit: use NLA_POLICY for parsing 'ex' keys

Transform two checks in the 'ex' key parsing into netlink policies
removing extra if checks.

Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>