pagecache can be reclaimed via the page LRU and via prune_icache.  We
currently don't know how much reclaim is happening via each.

The patch adds instrumentation to display the number of pages which were
freed via prune_icache.  This is displayed in /proc/vmstat:pginodesteal and
/proc/vmstat:kswapd_inodesteal.

Turns out that under some workloads (well, dbench at least), fully half of
page reclaim is via the unused inode list.  Which seems quite OK to me.