ospf6d : Code refactoring for route redistribution.

1. Created new ospf6_redist structure.
2. Moved the 'route_map' structure from structure 'ospf6' to
   structure 'ospf6_redist'.
3. Added new message type OSPF6_REDISTRIBUTE.
4. Added the placeholder for metric option in structure ospf6_redist
   for redistribute.
5. Added few API's for route redistribute lookup, add & del.

Signed-off-by: Kaushik <kaushik@niralnetworks.com>