BUG/MINOR: freq_ctr: use stricter barriers between updates and readings

BUG/MINOR: freq_ctr: use stricter barriers between updates and readings

update_freq_ctr_period() was using relaxed atomics without using barriers,
which usually works fine on x86 but not everywhere else. In addition, some
values were read without being enclosed by barriers, allowing the compiler
to possibly prefetch them a bit earlier. Finally, freq_ctr_total() was also
reading these without enough barriers. Let's make explicit use of atomic
loads and atomic stores to get rid of this situation. This required to
slightly rearrange the freq_ctr_total() loop, which could possibly slightly
improve performance under extreme contention by avoiding to reread all
fields.

A backport may be done to 2.4 if a problem is encountered, but last tests
on arm64 with LSE didn't show any issue so this can possibly stay as-is.