btrfs: btrfs_dev_replace_cancel() can return int

Current u64 return from btrfs_dev_replace_cancel() was probably done
to match the btrfs_ioctl_dev_replace_args::result. However as our
actual return value fits in int, and it further gets typecast to u64,
so just return int.

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