mount: disable mount-storm protection while mount unit is starting.

The starting of mount units requires that changes to
/proc/self/mountinfo be processed before the SIGCHILD from the
completion of /sbin/mount is processed, as described by the comment
  /* Note that due to the io event priority logic, we can be sure the new mountinfo is loaded
   * before we process the SIGCHLD for the mount command. */

The recently-added mount-storm protection can defeat this as it
will sometimes deliberately delay processing of /proc/self/mountinfo.

So we need to disable mount-storm protection when a mount unit is starting.
We do this by keeping a counter of the number of pending
mounts, and disabling the protection when this is non-zero.

Thanks to @asavah for finding and reporting this problem.