Staging: pohmelfs: distributed locking and cache coherency protocol.

POHMELFS utilizes writeback cache, which is built on top of MO(E)SI-like
coherency protocol. This patch includes its implementation and cache
object processing helpers (like allocation and completion callbacks).

POHMELFS uses scalable cached read/write locking. No additional requests
are performed if lock is granted to the filesystem. The same protocol
is used by the server to on-demand flushing of the client's cache (for
example when server wants to update local data).

Signed-off-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>