tc/mqprio: add support for preemptible traffic classes

tc/mqprio: add support for preemptible traffic classes

Add support for the "fp" argument in tc-mqprio, which takes an array
of letters "E" (for express) or "P" (for preemptible), one per traffic
class, and transforms them into TCA_MQPRIO_TC_ENTRY_FP u32 attributes of
the TCA_MQPRIO_TC_ENTRY nest. We also dump these new netlink attributes
when they come from the kernel.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David Ahern <dsahern@kernel.org>