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>
