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>

common/dmthin

@@ -91,6 +91,8 @@ _dmthin_init()
 	if [ -z "$low_water" ]; then
 		low_water=204800	# 100M, in sectors
 	fi
+	# low_water is expressed in blocks of size $cluster_size
+	low_water=$((low_water / cluster_size))
 
 	# Need to make linear metadata and data devs.  From kernel docs:
 	# As a guide, we suggest you calculate the number of bytes to use in the