ANDROID: mm/vmalloc: Add override for lazy vunmap

Add an interface to disable lazy vunmap by forcing the threshold
to zero. This might be interesting for debugging/testing in general,
but primarily helps a horrible situation which needs to guarantee
that vmalloc aliases are up-to-date from atomic context, wherein
the only practical solution is to never let them get stale in
the first place.

Bug: 223346425
(cherry picked from commit 2a34c1503b85f49dd472dfd932dfcd16cab8ee8a
 https://git.gitlab.arm.com/linux-arm/linux-rm.git

 arm64/2454944-dev)
Change-Id: I12fbbe3903f76a028ceea91ed078f0de2abe3815
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[BM: Convert to a flag that can be explicitly modified at runtime
     instead of relying on arch specific bits]
Signed-off-by: Beata Michalska <beata.michalska@arm.com>