target: Fix unsupported WRITE_SAME sense payload

This patch fixes a bug in target-core where unsupported WRITE_SAME ops
from a target_check_write_same_discard() failure was incorrectly
returning CHECK_CONDITION w/ TCM_INVALID_CDB_FIELD sense data.
This was causing some clients to not properly fall back, so go ahead
and use the correct TCM_UNSUPPORTED_SCSI_OPCODE sense for this case.

Reported-by: Martin Svec <martin.svec@zoner.cz>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>