tracing/osnoise: Improve comments about barrier need for NMI callbacks

trace_osnoise_callback_enabled is used by ftrace_nmi_enter/exit()
to know when to call the NMI callback. The barrier is used to
avoid having callbacks enabled before the resetting date during
the start or to touch the values after stopping the tracer.

Link: https://lkml.kernel.org/r/a413b8f14aa9312fbd1ba99f96225a8aed831053.1635702894.git.bristot@kernel.org

Cc: Ingo Molnar <[email protected]>
Cc: Tom Zanussi <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Juri Lelli <[email protected]>
Cc: Clark Williams <[email protected]>
Cc: John Kacur <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Sebastian Andrzej Siewior <[email protected]>
Cc: Daniel Bristot de Oliveira <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Suggested-by: Steven Rostedt <[email protected]>
Signed-off-by: Daniel Bristot de Oliveira <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

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