selftest: mptcp: exit from copyfd_io_poll() when receive SIGUSR1

For now, mptcp_connect won't exit after receiving the 'SIGUSR1' signal
if '-r' is set. Fix this by skipping poll and sleep in copyfd_io_poll()
if 'quit' is set.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/mptcp/mptcp_connect.c

@@ -627,7 +627,7 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
 		char rbuf[8192];
 		ssize_t len;
 
-		if (fds.events == 0)
+		if (fds.events == 0 || quit)
 			break;
 
 		switch (poll(&fds, 1, poll_timeout)) {
@@ -733,7 +733,7 @@ static int copyfd_io_poll(int infd, int peerfd, int outfd,
 	}
 
 	/* leave some time for late join/announce */
-	if (cfg_remove)
+	if (cfg_remove && !quit)
 		usleep(cfg_wait);
 
 	return 0;