btrfs: reset device stat using btrfs_dev_stat_set

btrfs_dev_stat_reset() is an overdo in terms of wrapping. So this patch
open codes btrfs_dev_stat_reset().

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>