tracing: Make tracing_selftest_running global to the tracing subsystem

tracing: Make tracing_selftest_running global to the tracing subsystem

The file trace.c has become a catchall for most things tracing. Start
making it smaller by breaking out various aspects into their own files.

Make the variable tracing_selftest_running global so that it can be used
by other files in the tracing subsystem and trace.c can be split up.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://patch.msgid.link/20260208032449.648932796@kernel.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>