net: ipv6: Do not add same prefix multiple times to timer list

Make sure that we do not add same IPv6 prefix, received from RA,
multiple times to prefix timer list. This avoids possible
denial-of-service issue if we receive suitably crafted RA packet.

Fixes #25698

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