FROMGIT: mm/vmalloc: Add override for lazy vunmap

Add an interface for arch code 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)
Change-Id: I694523564357b4c43d30c129af1e89fd803824d3
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Beata Michalska <beata.michalska@arm.com>