btrfs: define support masks for ioctl volume args v2

The ioctl data for devices or subvolumes can be passed via
btrfs_ioctl_vol_args or btrfs_ioctl_vol_args_v2. The latter is more
versatile and needs some caution as some of the flags make sense only
for some ioctls.

As we're going to extend the flags, define support masks for each ioctl
class separately.

Reviewed-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>