cachestat: do not flush stats in recency check

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: cachestat: do not flush stats in recency check syzbot detects that cachestat() is flushing stats, which can sleep, in its RCU read section (see [1]). This is done in the workingset_test_recent() step (which checks if the folio's eviction is recent). Move the stat flushing step to before the RCU read section of cachestat, and skip stat flushing during the recency check. [1]: https://lore.kernel.org/cgroups/000000000000f71227061bdf97e0@google.com/

References

• https://git.kernel.org/stable/c/e2f7c76758be16f1dc32c5a82270d4f6649eedab
• https://git.kernel.org/stable/c/1d1ba14e00d290b1ed616ed78c8c49bf897ce390
• https://git.kernel.org/stable/c/5a4d8944d6b1e1aaaa83ea42c116b520b4ed0394