CVE-2024-41033 cachestat: do not flush stats in recency check

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.