net/sched: mqprio: refactor nlattr parsing to a separate function

[ Upstream commit feb2cf3dcfb930aec2ca65c66d1365543d5ba943 ]

mqprio_init() is quite large and unwieldy to add more code to.
Split the netlink attribute parsing to a dedicated function.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: 6c58c8816abb ("net/sched: mqprio: Add length check for TCA_MQPRIO_{MAX/MIN}_RATE64")
Signed-off-by: Sasha Levin <sashal@kernel.org>