CIFS: Make use of common cifs_build_path_to_root for CIFS and SMB2

because the is no difference here. This also adds support of prefixpath
mount option for SMB2.

Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>