cifs: enable signing flag in SMB header when server has it on

cifs_sign_smb only generates a signature if the correct Flags2 bit is
set. Make sure that it gets set correctly if we're sending an async
call.

This patch fixes:

    https://bugzilla.kernel.org/show_bug.cgi?id=28142

Reported-and-Tested-by: JG <jg@cms.ac>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/transport.c

@@ -359,6 +359,10 @@ cifs_call_async(struct TCP_Server_Info *server, struct smb_hdr *in_buf,
 	if (rc)
 		return rc;
 
+	/* enable signing if server requires it */
+	if (server->secMode & (SECMODE_SIGN_REQUIRED | SECMODE_SIGN_ENABLED))
+		in_buf->Flags2 |= SMBFLG2_SECURITY_SIGNATURE;
+
 	mutex_lock(&server->srv_mutex);
 	mid = AllocMidQEntry(in_buf, server);
 	if (mid == NULL) {