dir: generic dummy quota support
dir: generic dummy quota support

This implements StorageEntitySetQuota() which is a storage volume type agnostic
replacement and generalisation of ContainerSetQuota().
In the case of the storage dir driver this is a noop.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

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