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.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>