ceph: support ceph_pagelist for message payload

The ceph_pagelist is a simple list of whole pages, strung together via
their lru list_head.  It facilitates encoding to a "buffer" of unknown
size.  Allow its use in place of the ceph_msg page vector.

This will be used to fix the huge buffer preallocation woes of MDS
reconnection.

Signed-off-by: Sage Weil <sage@newdream.net>

diff --cc fs/ceph/Makefile
Simple merge

diff --cc fs/ceph/messenger.c
Simple merge

diff --cc fs/ceph/messenger.h
Simple merge

diff --cc fs/ceph/pagelist.c
Simple merge

diff --cc fs/ceph/pagelist.h
Simple merge