tracing/stat: remove unappropriate safe walk on list

register_stat_tracer() uses list_for_each_entry_safe
to check whether a tracer is already present in the list.
But we don't delete anything from the list here, so
we don't need the safe version

[ Impact: cleanup list use is stat tracing ]

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>