[Staking] migrate_currency fails in some scenarios

[Ank4n]

This issue was initially reported by Kami | Polkagate.

The migrate_currency extrinsic expects the reserved amount to be zero and the locked funds to match the total staked amount in order for the migration to succeed. However, this assumption doesn’t always hold — if the account has updated its stake (e.g., through bonding, unbonding, or withdrawing), these values can diverge, causing migrate_currency to fail.