ospf6d: fix selection of NSSA forwarding address

ospf6d: fix selection of NSSA forwarding address

Change ospf6_get_nssa_fwd_addr() to try finding a global address
on any interface of the area and not on the first one only.

Additionally, do a micro-optimization in
ospf6_interface_get_global_address() to return as soon as a global
address is found.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>