shutdown: also fsync() MD devices when going down

Let's make this explicit, just in case this suffers by the same issues
as the loopback devices, and drops in-flight IO when we disassemble it.

src/shutdown/umount.c

@@ -484,6 +484,9 @@ static int delete_md(MountPoint *m) {
         if (fd < 0)
                 return -errno;
 
+        if (fsync(fd) < 0)
+                log_debug_errno(errno, "Failed to sync MD block device %s, ignoring: %m", m->path);
+
         if (ioctl(fd, STOP_ARRAY, NULL) < 0)
                 return -errno;