net: rpl: RPL route entry was fetched too late

The RPL route entry variable "extra" was resolved inside an
if-statement and the code was always returning from it.
This meant the the later code dealing with "extra" was never
executed. Fixed by moving the resolving of "extra" a bit
earlier so that the "extra" variable has always a proper value.

Coverity-CID: 173659
Coverity-CID: 173654

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>