[CIFS] Turn off Unicode during session establishment for plaintext authentication

LANMAN session setup did not support Unicode (after session setup, unicode can
still be used though).

Fixes samba bug# 5319

CC: Jeff Layton <jlayton@redhat.com>
CC: Stable Kernel <stable@vger.kernel.org>
Signed-off-by: Steve French <sfrench@us.ibm.com>