ipv6: Remove external dependency on rt6i_dst and rt6i_src

ipv6: Remove external dependency on rt6i_dst and rt6i_src

This patch removes the assumptions that the returned rt is always
a RTF_CACHE entry with the rt6i_dst and rt6i_src containing the
destination and source address.  The dst and src can be recovered from
the calling site.

We may consider to rename (rt6i_dst, rt6i_src) to
(rt6i_key_dst, rt6i_key_src) later.

Signed-off-by: Martin KaFai Lau <[email protected]>
Reviewed-by: Hannes Frederic Sowa <[email protected]>
Cc: Steffen Klassert <[email protected]>
Cc: Julian Anastasov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>