target: Fix assignment of LUN in tracepoints

The unpacked_lun field in the SCSI target tracepoints should be
initialized with cmd->orig_fe_lun rather than cmd->se_lun->unpacked_lun
for two reasons:

 - most importantly, if we are in the cmd_complete tracepoint
   returning a check condition due to no LUN found, cmd->se_lun will
   be NULL and we'll crash trying to dereference it.

 - also, in any case, cmd->se_lun->unpacked_lun is an internal index
   into the target's internal set of LUNs; cmd->orig_fe_lun is much
   more useful and interesting, since it's the value the initiator
   actually sent.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Cc: <stable@vger.kernel.org> # 3.11+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>