Now the LRUs are per-zone, make their lock per-zone as well.

In this patch the per-zone lock shares a cacheline with the zone's
buddy list lock, which is very bad.  Some groundwork is needed to fix
this well.

This change is expected to be a significant win on NUMA, where most
page allocation comes from the local node's zones.

For NUMA the `struct zone' itself should really be placed in that
node's memory, which is something the platform owners should look at.
However the internode cache will help here.

Per-node kswapd would make heaps of sense too.