net: rpl: DAO message was discarded too early

The commit b14586c3 ("net: rpl: RPL route entry was fetched
too late") dropped the DAO packet too early which prevents the
RPL root node functionality. Rework the earlier commit so that
Coverity issues are fixed but the RPL DAO message is also properly
handled.

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