This allows us to control the aggressiveness of the lower-zone defense
algorithm.  The `incremental min'.  For workloads which are using a
serious amount of mlocked memory, a few megabytes is not enough.

So the `lower_zone_protection' tunable allows the administrator to
increase the amount of protection which lower zones receive against
allocations which _could_ use higher zones.

The default value of lower_zone_protection is zero, giving unchanged
behaviour.  We should not normally make large amounts of memory
unavailable for pagecache just in case someone mlocks many hundreds of
megabytes.