[optimization]optimize TarStream for overlay2

[firjest]

Signed-off-by: yifang.jy yifang.jy@alibaba-inc.com

- What I did
The tarstream actually only needs the data of the current layer, and if the driver's get function is called, it is too heavy. Especially in the business customization process, if use remote disk as lowerdir(in some company like us), the get function needs to prepare the remote disk first and then mount it into overlay2. This process is very long and the remote disk will eventually not be used.

This is a hack way, is there a simpler or more general way to optimize?

This is just an example, for demonstration purposes.
- How I did it

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

@kolyshkin @dmcgowan @tonistiigi

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5ddb1d4). Click here to learn what that means.
The diff coverage is 38.88%.

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[tonistiigi]

Just define DiffGetter() on the driver and layerstore will pick it up automatically. Eg. look at aufs implementation.

Not 100% sure if this is safe for overlay though, in case of the special xattrs on files.

[thaJeztah]

Not 100% sure if this is safe for overlay though, in case of the special xattrs on files.

ping @dmcgowan ptal