ceph: preallocate buffer for readdir reply

Preallocate buffer for readdir reply. Limit number of entries in
readdir reply according to the buffer size.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>

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

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

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