mm: vmpressure: Don't cache the window size

Caching the window size can result in delayed or inaccurate pressure
reports. Since calculating a fresh window size is cheap, do so all the
time instead of relying on a stale, cached value.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Danny Lin <danny@kdrag0n.dev>