scsi: target: use ISCSI_IQN_LEN in iscsi_target_stat

Move the ISCSI_IQN_LEN definition up, so that it can be used in more
places instead of a hardcoded value.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>