target: Fix WRITE_SAME_[16,32] number of blocks=0 case

This patch fixes the handling of WRITE_SAME_[16,32] emulation where a
WRITE_SAME_* CDB with number of blocks=0 was being rejected by SCSI
expected data transfer length overflow checking in target core.

It changes both CDB cases in transport_generic_cmd_sequencer() to use
dev->se_sub_dev->se_dev_attrib.block_size to match what sg_write_same
is sending us with --num=0.  It also fixes target_emulate_write_same()
to properly determine the num_blocks with --num=0 case to determine the
remaining range for dev->transport->do_discard().

Reported-by: Chris Greiveldinger <chris.greiveldinger@rnanetworks.com>
Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>