Reorganize sequence of reestablishment checks

Reorganize the sequence of checks done in reestablishment to make it
more logical:

- check conditions of request (correct C-RNTI)
- check conditions of DU (has MIB/SIB1, MTC)
- check the UE, and from which DU it comes

This will also help to extend it towards reestablishment coming from
different DUs in the case of handover.