net: rpl: Update RPL header

Empty RPL HBH header will be inserted while finalizing IPv6 packet
but updated after finding nexthop and sent the packet. In case of
Bluetooth or multicast dst address it was missed. Resulted in
empty RPL HBH header and packet dropped at peer node. It should
be updated in all circumstances.

Jira: ZEP-2088

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>