block/iscsi: fix memory corruption on iscsi resize

bs->total_sectors is not yet updated at this point. resulting
in memory corruption if the volume has grown and data is written
to the newly availble areas.

CC: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
