ceph: specify max_bytes on readdir replies

Specify max bytes in request to bound size of reply.  Add associated
mount option with default value of 512 KB.

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

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

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

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

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