tracing: Fix race with update_max_tr_single and changing tracers

The commit 34600f0e

 "tracing: Fix race with max_tr and changing tracers"
fixed the updating of the main buffers with the race of changing
tracers, but left out the fix to the updating of just a per cpu buffer.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>