[CIFS] Warn if SMB3 encryption required by server

We do not support SMB3 encryption yet, warn if server responds
that SMB3 encryption is mandatory.

Signed-off-by: Steve French <smfrench@gmail.com>

diff --cc fs/cifs/smb2pdu.c
Simple merge

diff --cc fs/cifs/smb2pdu.h
Simple merge