dm thin metadata: use true/false for bool variable

Fixes coccicheck warning:

drivers/md/dm-thin-metadata.c:814:3-14: WARNING: Assignment of 0/1 to bool variable
drivers/md/dm-thin-metadata.c:1109:1-12: WARNING: Assignment of 0/1 to bool variable
drivers/md/dm-thin-metadata.c:1621:1-12: WARNING: Assignment of 0/1 to bool variable
drivers/md/dm-thin-metadata.c:1652:1-12: WARNING: Assignment of 0/1 to bool variable
drivers/md/dm-thin-metadata.c:1706:1-12: WARNING: Assignment of 0/1 to bool variable

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>