tracelog: use same class for all loglevels

tracelog: use same class for all loglevels

Each loglevel can be serialized by the exact same serialization
function. Therefore, use a single tracepoint class for all those.

This diminishes the code size of the probe, and therefore its impact on
the instruction cache.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>