MARK SMB2 support EXPERIMENTAL

Now that the merge of the remaining pieces needed for
SMB2 (SMB2.1 dialect) are in, and most test cases pass,
we can consider SMB2.1 EXPERIMENTAL rather than "BROKEN."

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