packets: Cleanup ND compose functions.

packets: Cleanup ND compose functions.

Rename "compose_nd" and "compose_na" to "compose_nd_ns" and
"compose_nd_na", respecively, to be clearer about their functionality.
This will also make it more consistent when we add Neighbor Discover
Router Solicitation/Advertisement compose functions.

Also change the source and destination IPv6 addresses to take
"struct in6_addr" arguments, which are more common in the code base.

Signed-off-by: Justin Pettit <jpettit@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>