block/iscsi: always fill bs->bl.opt_transfer_length

the opt_transfer_length has nothing to do with logical
block provisioning stuff so always copy it from
the block limits VPD page.

Reported-By: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
