cifs: don't start signing too early

Sniffing traffic on the wire shows that windows clients send a zeroed
out signature field in a NEGOTIATE request, and send "BSRSPYL" in the
signature field during SESSION_SETUP. Make the cifs client behave the
same way.

It doesn't seem to make much difference in any server that I've tested
against, but it's probably best to follow windows behavior as closely as
possible here.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>