Spend the right number of passes in slot_testv_and_readv_and_writev when leases have expired

If there are some shares in a slot and the leases have expired on those shares, when a client decides how much to spend to do a write to that slot it considers the size of the shares but ignores the fact that they have no valid leases.

The server considers the expired leases, though, and believes the entire share needs to be paid for in order to allow a write. So the client underpays and the server rejects the write.

Make the client consider the state of leases when deciding how much to spend.