tc: Add helpers to support qevent handling

tc: Add helpers to support qevent handling

Introduce a set of helpers to make it easy to add support for qevents into
qdisc.

The idea behind this is that qevent types will be generally reused between
qdiscs, rather than each having a completely idiosyncratic set of qevents.
The qevent module holds functions for parsing, dumping and formatting of
these common qevent types, and for dispatch to the appropriate set of
handlers based on the qevent name.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David Ahern <dsahern@kernel.org>