Make it possible for applications to have the kernel free memory
lazily.  This reduces a repeated free/malloc cycle from freeing
pages and allocating them, to just marking them freeable.  If the
application wants to reuse them before the kernel needs the memory,
not even a page fault will happen.

This patch, together with Ulrich's glibc change, increases
MySQL sysbench performance by a factor of 2 on my quad core
test system.

Signed-off-by: Rik van Riel <riel@redhat.com>

---
Ulrich Drepper has test glibc RPMS for this functionality at:

     http://people.redhat.com/drepper/rpms

Andrew, I have stress tested this patch for a few days now and
have not been able to find any more bugs.  I believe it is ready
to be merged in -mm, and upstream at the next merge window.

When the patch goes upstream, I will submit a small follow-up
patch to revert MADV_DONTNEED behaviour to what it did previously
and have the new behaviour trigger only on MADV_FREE: at that
point people will have to get new test RPMs of glibc.