mm/page_alloc.c: calculate 'available' memory in a separate function

mm/page_alloc.c: calculate 'available' memory in a separate function

Add a new field, VIRTIO_BALLOON_S_AVAIL, to virtio_balloon memory
statistics protocol, corresponding to 'Available' in /proc/meminfo.

It indicates to the hypervisor how big the balloon can be inflated
without pushing the guest system to swap.  This metric would be very
useful in VM orchestration software to improve memory management of
different VMs under overcommit.

This patch (of 2):

Factor out calculation of the available memory counter into a separate
exportable function, in order to be able to use it in other parts of the
kernel.

In particular, it appears a relevant metric to report to the hypervisor
via virtio-balloon statistics interface (in a followup patch).

Signed-off-by: Igor Redko <redkoi@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit d02bd27bd33dd7e8d22594cd568b81be0cb584cd)

Orabug: 30073695

Need this to provide si_mem_available() for the next patch

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Patrick Colp <patrick.colp@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	fs/proc/meminfo.c: we don't have commit 84ad5802a33a ("proc:
        meminfo: estimate available memory more conservatively") and
        even though it looks reasonable and simple I didn't want to
        include it because this would change a user-visible attribute.

Signed-off-by: Brian Maly <brian.maly@oracle.com>