net_sched actions: use nla_parse_nested()

Use nla_parse_nested instead of open-coding the call to
nla_parse() with the attribute data/len.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>