ceph: fix memory leak in ceph_readdir when note_last_dentry returns error

Reset the last_readdir at the same time, and add a comment explaining
why we don't free last_readdir when dir_emit returns false.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>