ieee1394: sbp2: offer SAM-conforming target port ID in sysfs

With "modprobe sbp2 long_ieee1394_id=y", the format of
/sys/bus/scsi/devices/*:*:*:*/ieee1394_id is changed from e.g.
0001041010004beb:0:0 to 0001041010004beb:00042c:0000.

The longer format fully conforms to object identifier sizes as per
SAM(-2...4) and reflects what the SAM target port identifier is meant to
contain:  A Discovery ID allegedly specified by ISO/IEC 13213:1994 ---
however there is no such thing; the authors of SAM probably meant
Directory ID).  Especially target nodes with multiple dynamically added
targets may use Directory IDs to persistently identify target ports.

The new format is independent of implementation details of nodemgr.
Thus the same ieee1394_id attribute format can be implemented in the new
firewire stack.

The ieee1394_id is typically used to create persistently named links in
/dev/disk/by-id.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>