block/iscsi: clarify the meaning of ISCSI_CHECKALLOC_THRES

Signed-off-by: Peter Lieven <pl@kamp.de>
block/iscsi: allow cluster_size of 4K and greater

depending on the target the opt_unmap_gran might be as low
as 4K. As we know use this also as a knob to activate the allocationmap
feature lower the barrier. The limit 4K (and not 512) is choosen
to avoid a potentially too big allocationmap.

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