dm snapshot: use true/false for bool variable

Fixes coccicheck warning:

drivers/md/dm-snap.c:1064:3-18: WARNING: Assignment of 0/1 to bool variable
drivers/md/dm-snap.c:1152:1-16: WARNING: Assignment of 0/1 to bool variable
drivers/md/dm-snap.c:1317:1-16: 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>