rpl: fix offset for transit buf

As a result of PR #1404, entries will not be added to the routing table
when running in storing mode, although it is supposed to do so.
You can verify by running rpl_udp and call *route* on the dodag-root.

This PR fixes the wrong offset calculation.
Merge pull request #2145 from cgundogan/rpl_fix_transit_buf_offset

rpl: fix offset for transit buf