Fix space check in FileWriter device list for mount points of more than one directory level

Filesystems mounted on a subdirectory of more than one level of "/" were not properly detected. Also simplify the code a bit.

Finding the device with the most free space doesn't seem to be used in production any more, as the only usage of the FileWriter that I could find is in the align writers and they all have `/calib' as their device list. The check could therefore also be removed.