mm: vmpressure: allow in-kernel clients to subscribe for events

Currently, vmpressure is tied to memcg and its events are
available only to userspace clients. This patch removes
the dependency on CONFIG_MEMCG and adds a mechanism for
in-kernel clients to subscribe for vmpressure events (in
fact raw vmpressure values are delivered instead of vmpressure
levels, to provide clients more flexibility to take actions
on custom pressure levels which are not currently defined
by vmpressure module).

Change-Id: I38010f166546e8d7f12f5f355b5dbfd6ba04d587
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>