target: Fix missing length check in spc_emulate_evpd_83()

Commit fbfe858fea2a ("target_core_spc: Include target device
descriptor in VPD page 83") added a new length variable, but (due to a
cut and paste mistake?) just checks scsi_name_len against 256 twice.
Fix this to check scsi_target_len for overflow too.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>