tracepoints: Code clean up

Restructure the code a bit to make it simpler, fix some formatting problems
and add READ_ONCE/WRITE_ONCE to make sure there's no compiler load/store
tearing to the variables that can be accessed across CPUs.

Started with Mathieu Desnoyers's patch:

  Link: https://lore.kernel.org/lkml/20210203175741.20665-1-mathieu.desnoyers@efficios.com/

And will keep his signature, but I will take the responsibility of this
being correct, and keep the authorship.

Link: https://lkml.kernel.org/r/20210204143004.61126582@gandalf.local.home



Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>