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.

[1]: https://lore.kernel.org/cgroups/[email protected]/