net/sched: sch_mqprio: use netlink payload helpers

net/sched: sch_mqprio: use netlink payload helpers

For the sake of readability, use the netlink payload helpers from
the 'nla_get_*()' family to parse the attributes.

tdc results:
1..5
ok 1 9903 - Add mqprio Qdisc to multi-queue device (8 queues)
ok 2 453a - Delete nonexistent mqprio Qdisc
ok 3 5292 - Delete mqprio Qdisc twice
ok 4 45a9 - Add mqprio Qdisc to single-queue device
ok 5 2ba9 - Show mqprio class

Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230404203449.1627033-1-pctammela@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>