The remaining source of page-at-a-time activity against
pagemap_lru_lock is the anonymous pagefault path, which cannot be
changed to operate against multiple pages at a time.

But what we can do is to batch up just its adding of pages to the LRU,
via buffering and deferral.

This patch is based on work from Bill Irwin.

The patch changes lru_cache_add to put the pages into a per-CPU
pagevec.  They are added to the LRU 16-at-a-time.

And in the page reclaim code, purge the local CPU's buffer before
starting.  This is mainly to decrease the chances of pages staying off
the LRU for very long periods: if the machine is under memory pressure,
CPUs will spill their pages onto the LRU promptly.

A consequence of this change is that we can have up to 15*num_cpus
pages which are not on the LRU.  Which could have a slight effect on VM
accuracy, but I find that doubtful.  If the system is under memory
pressure the pages will be added to the LRU promptly, and these pages
are the most-recently-touched ones - the VM isn't very interested in
them anyway.

This optimisation could be made SMP-specific, but I felt it best to
turn it on for UP as well for consistency and better testing coverage.