Use the same adaptive readdirplus mechanism as NFS:

http://permalink.gmane.org/gmane.linux.nfs/49299

If the user space implementation wants to disable readdirplus
temporarily, it could just return ENOTSUPP. Then kernel will
recall it with readdir.
Signed-off-by: Feng Shuo <steve.shuo.feng@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>