[iscsi] Send any padding inline with the data segment

[iscsi] Send any padding inline with the data segment

Some iSCSI targets respond to a PDU before receiving the padding
bytes.  If the target responds quickly enough, this can cause iPXE to
start processing a new TX PDU before the padding bytes have been sent,
which results in a protocol violation.

Fix by always transmitting the padding bytes along with the data
segment.

Originally-fixed-by: Shyam Iyer <shyam_iyer@dell.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>