net/sched: mqprio: simplify handling of nlattr portion of TCA_OPTIONS

In commit 4e8b86c0 ("mqprio: Introduce new hardware offload mode and
shaper in mqprio"), the TCA_OPTIONS format of mqprio was extended to
contain a fixed portion (of size NLA_ALIGN(sizeof struct tc_mqprio_qopt))
and a variable portion of other nlattrs (in the TCA_MQPRIO_* type space)
following immediately afterwards.

In commit feb2cf3d ("net/sched: mqprio: refactor nlattr parsing to a
separate function"), we've moved the nlattr handling to a smaller
function, but yet, a small parse_attr() still remains, and the larger
mqprio_parse_nlattr() still does not have access to the beginning, and
the length, of the TCA_OPTIONS region containing these other nlattrs.

In a future change, the mqprio qdisc will need to iterate through this
nlattr region to discover other attributes, so eliminate parse_attr()
and add 2 variables in mqprio_parse_nlattr() which hold the beginning
and the length of the nlattr range.

We avoid the need to memset when nlattr_opt_len has insufficient length
by pre-initializing the table "tb".

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ferenc Fejes <fejes@inf.elte.hu>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>