lxd/storage/drivers/driver/btrfs/volumes: Enable nodatacow on subvolume for block volumes
lxd/storage/drivers/driver/btrfs/volumes: Enable nodatacow on subvolume for block volumes

This reduces the impact of random writes to large block files on the BTRFS quota.
Although this does not entirely prevent CoW from taking up quota usage, as when a snapshot is taken,
the first write to a block after that will cause a CoW and the new and old extent will be referenced
as used in the quota

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

hallyn/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,lxc/lxd