int flex_array_shrink(struct flex_array *fa)

This function will free all unused second-level pages.  Since elements are
now poisoned if they are not allocated with __GFP_ZERO, it's possible to
identify parts that consist solely of unused elements.

flex_array_shrink() returns the number of pages freed.
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>