fix mdmon takeover

- when we waited for the old mdmon to exit, we didn't look
  for the socket in the right place

- when we failed to find a pid file, we returned the wrong
  value (code expected <0, but got ==0).

Signed-off-by: Luca Berra <bluca@comedia.it>
Signed-off-by: NeilBrown <neilb@suse.de>

msg.c

@@ -147,7 +147,7 @@ int connect_monitor(char *devname)
 	int pos;
 	char *c;
 
-	pos = sprintf(path, "%s/", VAR_RUN);
+	pos = sprintf(path, "%s/", pid_dir);
 	if (is_subarray(devname)) {
 		devname++;
 		c = strchr(devname, '/');

util.c

@@ -1478,11 +1478,11 @@ int mdmon_pid(int devnum)
 	fd = open(path, O_RDONLY | O_NOATIME, 0);
 
 	if (fd < 0)
-		return 0;
+		return -1;
 	n = read(fd, pid, 9);
 	close(fd);
 	if (n <= 0)
-		return 0;
+		return -1;
 	return atoi(pid);
 }