ceph: simplify ceph_buffer interface

We never allocate the ceph_buffer and buffer separtely, so use a single
constructor.

Disallow put on NULL buffer; make the caller check.

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

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

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

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

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

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