CIFS: Fix SMB3 mount without specifying a security mechanism

Commit ef65aaede23f ("smb2: Enforce sec= mount option") changed the
behavior of a mount command to enforce a specified security mechanism
during mounting. On another hand according to the spec if SMB3 server
doesn't respond with a security context it implies that it supports
NTLMSSP. The current code doesn't keep it in mind and fails a mount
for such servers if no security mechanism is specified. Fix this by
indicating that a server supports NTLMSSP if a security context isn't
returned during negotiate phase. This allows the code to use NTLMSSP
by default for SMB3 mounts.

Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/smb2pdu.c

@@ -562,8 +562,10 @@ SMB2_negotiate(const unsigned int xid, struct cifs_ses *ses)
 	 * but for time being this is our only auth choice so doesn't matter.
 	 * We just found a server which sets blob length to zero expecting raw.
 	 */
-	if (blob_length == 0)
+	if (blob_length == 0) {
 		cifs_dbg(FYI, "missing security blob on negprot\n");
+		server->sec_ntlmssp = true;
+	}
 
 	rc = cifs_enable_signing(server, ses->sign);
 	if (rc)