net: if: Do not add IPv6 address lifetime timer twice to list

The IPv6 address lifetime timers are kept in a slist, but
the code that adds the entry to the list does not check
whether the item is already in the list. This will cause
problems when trying to remove the address from the list.

Normally this is not causing issues, but if the function
net_if_ipv6_addr_update_lifetime() is called multiple times
before the address expires, then the item was added to the
slist multiple times.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>