"double free" memory corruption in smbfs

smbfs allocates rq_trans2buffer to handle server's multi transaction2 response messages. As struct smb_request may be reused, rq_trans2buffer is freed before each new request. However if last servers's response is not multi but single trans2 message then new rq_trans2buffer is not allocated but last smb_rput still tries to free it again.
To prevent this issue rq_trans2buffer pointer should be set to NULL after kfree.

@@ -181,6 +181,7 @@ static int smb_setup_request(struct smb_
 	req->rq_errno = 0;
 	req->rq_fragment = 0;
 	kfree(req->rq_trans2buffer);
+	req->rq_trans2buffer = NULL;
 
 	return 0;
 }