debian: Fix lizardfs-master.init script

This script was checking for a condition which no longer makes sense:

  [[ -e /var/lib/mfs/metadata.mfs.back ]]

To avoid confusion this commit modifies it to something that would work
better with new format of data directory, ie:

  [[ -e /var/lib/mfs/metadata.mfs.lock ]]

Change-Id: I8c6c7534424eba410202489a8b313da5f3f4ace6