iscsi-test: check for invalid descriptors offsets in get_lba_status

Dell Equallogic sans report an invalid LBA offset in the descriptor when
using 4k LUNs. Check the descriptor for consistency with the request.

Signed-off-by: Peter Lieven <pl@kamp.de>
Merge pull request #125 from plieven/iscsi-test

iscsi-test: check for invalid descriptors offsets in get_lba_status