Patch from Bill Irwin.

It removes the custom pte_chain allocator in mm/rmap.c and replaces it
with a slab cache.

"This patch
 (1) eliminates the pte_chain_freelist_lock and all contention on it
 (2) gives the VM the ability to recover unused pte_chain pages

 Anton Blanchard has reported (1) from prior incarnations of this patch.
 Craig Kulesa has reported (2) in combination with slab-on-LRU patches.

 I've left OOM detection out of this patch entirely as upcoming patches
 will do real OOM handling for pte_chains and all the code changed anyway."