net: ipv6: nbr: Avoid casting unaligned address to struct in6_addr

IPv6 Neighbor Discovery interfaces modules like neighbor or routing
tables - converting them to raw variants seems futile. Therefore, for
IPv6 ND case, copy the raw IP address from the packet into the in6_addr
structure, and then pass it to respective functions. Performance
overhead should not be a big problem in such case as those actions are
only performed if a respective ND packet is received.

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