udev-rules: make tape-changers also apprear in /dev/tape/by-path/

It is important to be able to access tape changer ("Medium Changers") by
persistant name.
While tape devices can be accessed via /dev/tape/by-id/ and
/dev/tape/by-path/, tape-changers could only be accessed by
/dev/tape/by-id/.
However, in some cases, especially when accessing Amazon Webservice
Storage Gateway VTLs (or accessing iSCSI VTLs in general?) this does not
work, as all tape devices and the tape changer have the same ENV{ID_SERIAL}.
The results is, that only the last device is available in
/dev/tape/by-id/, as the former devices have been overwritten.

As this behavior is hard to change without breaking consistentcy,
this additional device in /dev/tape/by-path/ can be used to access the medium changes.
The tape devices can also be accessed by this path.

The content of the directory will now look like:

  # SCSI tape device, rewind (unchanged)
  /dev/tape/by-path/$env{ID_PATH} -> ../../st*

  # SCSI tape device, no-rewind (unchanged)
  /dev/tape/by-path/$env{ID_PATH}-nst -> ../../nst*

  # SCSI tape changer device (newly added)
  /dev/tape/by-path/$env{ID_PATH}-changer -> ../../sg*

Tape devices and tape changer have different ID_PATHs.
SCSI tape changer get the suffix "-changer"
to make them better distinguishable from tape devices.

rules/60-persistent-storage-tape.rules

@@ -9,6 +9,9 @@ ENV{UDEV_DISABLE_PERSISTENT_STORAGE_RULES_FLAG}=="1", GOTO="persistent_storage_t
 SUBSYSTEM=="scsi_generic", SUBSYSTEMS=="scsi", ATTRS{type}=="8", IMPORT{program}="scsi_id --sg-version=3 --export --whitelisted -d $devnode", \
   SYMLINK+="tape/by-id/scsi-$env{ID_SERIAL}"
 
+SUBSYSTEM=="scsi_generic", SUBSYSTEMS=="scsi", ATTRS{type}=="8", IMPORT{builtin}="path_id", \
+  SYMLINK+="tape/by-path/$env{ID_PATH}-changer"
+
 SUBSYSTEM!="scsi_tape", GOTO="persistent_storage_tape_end"
 
 KERNEL=="st*[0-9]|nst*[0-9]", ATTRS{ieee1394_id}=="?*", ENV{ID_SERIAL}="$attr{ieee1394_id}", ENV{ID_BUS}="ieee1394"