mm: vmpressure: Interpret zero scanned pages as 100% pressure

When no pages are scanned, it usually means no zones were reclaimable
and nothing could be done. In this case, the reported pressure should be
100 to elicit help from any listeners. This fixes the vmpressure
framework not working when memory pressure is very high.

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