SMB3: Add support for multidialect negotiate (SMB2.1 and later)

With the need to discourage use of less secure dialect, SMB1 (CIFS),
we temporarily upgraded the dialect to SMB3 in 4.13, but since there
are various servers which only support SMB2.1 (2.1 is more secure
than CIFS/SMB1) but not optimal for a default dialect - add support
for multidialect negotiation.  cifs.ko will now request SMB2.1
or later (ie SMB2.1 or SMB3.0, SMB3.02) and the server will
pick the latest most secure one it can support.

In addition since we are sending multidialect negotiate, add
support for secure negotiate to validate that a man in the
middle didn't downgrade us.

Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
CC: Stable <stable@vger.kernel.org> # 4.13+