Not possible to reserve_transfer_assets back from Statemine to Kusama

[hbulgarini]

Companion issue for paritytech/polkadot#5233

All reserved funds moved from Kusama to Statemine can not make their way back to Kusama.

It is not possible to do a reserve_transfer_assets() from Statemine to Kusama because it does not have any trusted Reserves: https://github.com/paritytech/polkadot/blob/master/runtime/kusama/src/xcm_config.rs#L136

The amount is deposited in the Parent's sovereign account in Statemine/Statemint, but the UMP message fails in the relay chain.

The same applies for Statemint<>Polkadot & Westmint<>Westend.

[NachoPal]

This PR paritytech/polkadot#4549 will open the door for users to "lose" their funds getting them trapped in Kusama's sovereign account in Statemine. As explained above, since Statemine is not a reserve of Kusama, ReserveAssetDeposited will be dropped and the UMP message will fail with UntrustedReserveLocation.

To solve this issue, we will modify XcmReserveTransferFilter in Statemine from Everything to something like EverythingButParent, to prevent users to reserve transfers to Kusama.

Same applies for Statmint<>Polkadot and Westmint<>Westend.

[gilescope]

This we believe is prevented by #1169

[hbulgarini]

Prevented with: #1169