[CIFS] Missing flag on negprot needed for some servers to force packet signing

A related signature issue that I came across.
There's a bug in win2k that when NT error codes are not negotiated, the
server doesn't response that signatures are mandatory. Since there's
(currently) no way turn on signatures in such case, I had to force NT
error codes, so that this bug will not occur

Signed-off-by: Yehuda Sadeh Weinraub <Yehuda.Sadeh@expand.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>