check-weight: Disable total pov size check for mandatory extrinsics (#4571)

So in some pallets we like
[here](https://github.com/paritytech/polkadot-sdk/blob/5dc522d0/substrate/frame/session/src/lib.rs#L556)
we use `max_block` as return value for `on_initialize` (ideally we would
not).

This means the block is already full when we try to apply the inherents,
which lead to the error seen in #4559 because we are unable to include
the required inherents. This was not erroring before #4326 because we
were running into this branch:

https://github.com/paritytech/polkadot-sdk/blob/e4b89cc5/substrate/frame/system/src/extensions/check_weight.rs#L222-L224

The inherents are of `DispatchClass::Mandatory` and therefore have a
`reserved` value of `None` in all runtimes I have inspected. So they
will always pass the normal check.

So in this PR I adjust the `check_combined_proof_size` to return an
early `Ok(())` for mandatory extrinsics.

If we agree on this PR I will backport it to the 1.12.0 branch.

closes #4559

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Co-authored-by: command-bot <>