ceph: handle frag mismatch between readdir request and reply

If client has outdated directory fragments information, it may request
readdir an non-existent directory fragment. In this case, the MDS finds
an approximate directory fragment and sends its contents back to the
client. When receiving a reply with fragment that is different than the
requested one, the client need to reset the 'readdir offset'.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>

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

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

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