net: ipv6: Remove in6_addr from packed net_ipv6_hdr struct

Replace unpacked in6_addr structures with raw buffers in net_ipv6_hdr
struct, to prevent compiler warnings about unaligned access.

Remove __packed parameter from `struct net_6lo_context` since the
structure isn't really serialized.

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