mm: vmpressure: Don't export tunables to userspace

Userspace could change these tunables and make Simple LMK function
poorly. Don't export them.

Reported-by: attack11 <fernandobouchet@gmail.com>
Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Danny Lin <danny@kdrag0n.dev>