mount-util: Compact list of sub mounts after dropping

mount-util: Compact list of sub mounts after dropping

When nested mounts appear under a sysext hierarchy like this:
  mkdir -p /opt/trigger/
  mount -t tmpfs tmpfs /opt/trigger
  mkdir -p /opt/trigger/inner
  mount -t tmpfs tmpfs /opt/trigger/inner
Then systemd-sysext merge hit an assertion reported in
https://github.com/flatcar/Flatcar/issues/2111 because when it iterates
over the list of sub mounts it doesn't expect entries with NULL in the
path from the dropped entries.
Instead of having to deal with entries with path NULL, better sort the
holes from dropping to the end and then reduce the array length.