cifs: Fix signing failure when server mandates signing for NTLMSSP

When using NTLMSSP authentication mechanism, if server mandates
signing, keep the flags in type 3 messages of the NTLMSSP exchange
same as in type 1 messages (i.e. keep the indicated capabilities same).

Some of the servers such as Samba, expect the flags such as
Negotiate_Key_Exchange in type 3 message of NTLMSSP exchange as well.
Some servers like Windows do not.

https://bugzilla.samba.org/show_bug.cgi?id=8212



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