svcrpc: sends on closed socket should stop immediately

svc_tcp_sendto sets XPT_CLOSE if we fail to transmit the entire reply.
However, the XPT_CLOSE won't be acted on immediately.  Meanwhile other
threads could send further replies before the socket is really shut
down.  This can manifest as data corruption: for example, if a truncated
read reply is followed by another rpc reply, that second reply will look
to the client like further read data.

Symptoms were data corruption preceded by svc_tcp_sendto logging
something like

	kernel: rpc-srv/tcp: nfsd: sent only 963696 when sending 1048708 bytes - shutting down socket

Cc: stable@vger.kernel.org
Reported-by: Malahal Naineni <malahal@us.ibm.com>
Tested-by: Malahal Naineni <malahal@us.ibm.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/svc_xprt.c

@@ -794,7 +794,8 @@ int svc_send(struct svc_rqst *rqstp)
 
 	/* Grab mutex to serialize outgoing data. */
 	mutex_lock(&xprt->xpt_mutex);
-	if (test_bit(XPT_DEAD, &xprt->xpt_flags))
+	if (test_bit(XPT_DEAD, &xprt->xpt_flags)
+			|| test_bit(XPT_CLOSE, &xprt->xpt_flags))
 		len = -ENOTCONN;
 	else
 		len = xprt->xpt_ops->xpo_sendto(rqstp);