tracing: add TRACE_EVENT_MAP

tracing: add TRACE_EVENT_MAP

This macro allows to create an alias of an existing TRACE_EVENT. A
TRACE_EVENT_MAP connects a new probe to an existing tracepoint, so we
can use it to create another output of the same tracepoint without
changing the instrumented code.

This allows to create alternate versions of existing tracepoints to
output more/other fields only in specific use-cases and not all the time
(which could break existing tools and/or bloat the trace with too many
useless fields).

The usage is the same as the TRACE_EVENT macro with the addition of the
"map" parameter which is the name of the alias, the "name" field is the
name of the original tracepoint:
TRACE_EVENT_MAP(name, map, proto, args, tstruct, assign, print)
DEFINE_EVENT_MAP(template, name, map, proto, args)

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (Red Hat) <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>