target_core: simplify scsi_name_len calculation

scsi_name_len in spc_emulate_evpd_83 is calculated twice, with
the results of the first calculation discarded. So remove it.
And check for the maximum allowed length, too.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>