net: ipv6: Fix DAD conflict detection

Filtering out received packets which source address matches one of our
own addresses has broken the duplicate address detection conflict
reporting.

In case Neighbor Advertisement is sent during DAD collision, the source
address of such packet might be set to the address that triggered the
collision. This address would also be set on our interface, marked as
tentative. Therefore in order to make duplicate address detection work,
we cannot filter out packets in case our own address is still in
tentative state.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>