Fix SMB3.1.1 guest authentication to Samba

Samba rejects SMB3.1.1 dialect (vers=3.1.1) negotiate requests from
the kernel client due to the two byte pad at the end of the negotiate
contexts.

CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>