dmthin: express low_water in cluster_size not sector size

dmthin: express low_water in cluster_size not sector size

I noticed that thin-pool sent out "reached low water mark for data
device" event even before thin-pool device was mounted in
generic/347, this is because low water mark was set to a too high
value.

According to kernel thin-provisioning.txt documentation, low water
mark should be expressed in blocks of $cluster_size, not in sectors.

"$low_water_mark is expressed in blocks of size $data_block_size."

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>