common/dmthin: modify DMTHIN_POOL_NAME

common/dmthin: modify DMTHIN_POOL_NAME

_dmthin_set_queue() gets $cluster_size by 'dmsetup table | grep
$DMTHIN_POOL_NAME'. The original name 'thin-pool' is accidently
the same as the fourth field of the output of 'dmsetup table'.

For example,
vg-pool-tpool: 0 64864256 thin-pool 252:0 252:1 128 0 0

Thus if there is other thin-pool in the test environment, except
for the thin-pool created by _dmthin_init(), the parsed $cluster_size
may be a multi-line string, which is unexpected.

This issue caused a failure of generic/347 and generic/500, while the
dmesg reports 'Invalid argument count'.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>