block/iscsi: bump libiscsi requirement to 1.8.0

This patch lifts the minimum supported libiscsi version from 1.4.0 to
1.8.0 in preparation to an upcoming patch to dynamically switch between
10 Byte and 16 Byte CDBs.

On one this allows us to remove nearly all #ifdefs from the code which
makes the code easier to maintain and read. On the other hand
I would not recommend libiscsi prior to 1.8.0 for production use
because the following important libiscsi fixes for deadlocks and
protocol errors are missing prior to 1.8.0:

dbe9a1e SOCKET queue cmd PDUs directly in waitpdu queue
30df192 DATA-OUT set pdu->cmdsn appropriately
548bd22 ISCSI fix broken send logic in iscsi_scsi_async_command
14bee10 RECONNECT do not increase CmdSN for immediate PDUs
1f4a66a PDU queue out PDUs in order of itt.
562dd46 PDU avoid incrementing itt to 0xffffffff
cd09c0f PDU use serial32 arithmetic for cmdsn, maxcmdsn and expcmdsn.
89e918e SOCKET validate data_size in in_pdu header
91267f5 Limit immediate and unsolicited data to FirstBurstLength

Note that libiscsi 1.8.0 was already released Jan 5th, 2013.

Signed-off-by: Peter Lieven <pl@kamp.de>
block/iscsi: allow fall back to WRITE SAME without UNMAP

if the iscsi driver receives a write zeroes request with
the BDRV_REQ_MAY_UNMAP flag set it fails with -ENOTSUP
if the iscsi target does not support WRITE SAME with
UNMAP. However, the BDRV_REQ_MAY_UNMAP is only a hint
and writing zeroes with WRITE SAME will still be
better than falling back to writing zeroes with WRITE16.

Signed-off-by: Peter Lieven <pl@kamp.de>
block/iscsi: bump year in copyright notice

Signed-off-by: Peter Lieven <pl@kamp.de>
block/iscsi: use 16 byte CDBs only when necessary

this patch changes the driver to uses 16 Byte CDBs for
READ/WRITE only if the target requires 64bit lba addressing.

On one hand this saves 6 bytes in each PDU on the other
hand it seems that 10 Byte CDBs seems to be much better
supported and tested as a recent issue I had with a
major storage supplier lined out.

For WRITESAME the logic is a bit more tricky as WRITESAME10
with UNMAP was added really late. Thus a fallback to WRITESAME16
is possible if it supports UNMAP and WRITESAME10 not.

Signed-off-by: Peter Lieven <pl@kamp.de>