target: Fix off-by-seven in target_report_luns

cdb_offset is always equal to offset - 8, so remove that one.  More
importantly, the existing code only worked correct if
se_cmd->data_length is a multiple of 8.  Pass in a length of, say, 9 and
we will happily overwrite 7 bytes of "unallocated" memory.

Now, afaics this bug is currently harmless, as allocations will
implicitly be padded to multiples of 8 bytes.  But depending on such a
fact wouldn't qualify as sound engineering practice.

Signed-off-by: Joern Engel <joern@logfs.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>