tracing: Make tracing_disabled global for tracing system

tracing: Make tracing_disabled global for tracing system

The tracing_disabled variable is set to one on boot up to prevent some
parts of tracing to access the tracing infrastructure before it is set up.
It also can be set after boot if an anomaly is discovered.

It is currently a static variable in trace.c and can be accessed via a
function call trace_is_disabled(). There's really no reason to use a
function call as the tracing subsystem should be able to access it
directly.

By making the variable accessed directly, code can be moved out of trace.c
without adding overhead of a function call to see if tracing is disabled
or not.

Make tracing_disabled global and remove the tracing_is_disabled() helper
function. Also add some "unlikely()"s around tracing_disabled where it's
checked in hot paths.

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/20260208032449.483690153@kernel.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>