ospf6d: fix processing of inter-area-prefix-LSAs with the LA-bit set

ospf6d: fix processing of inter-area-prefix-LSAs with the LA-bit set

RFC 5340, Section 4.8.3 says:
  "Prefixes having the NU-bit set in their PrefixOptions field should
  be ignored by the inter-area route calculation".

Fix a bug where, in addition to the NU-bit, ospf6d was also ignoring
prefixes having the LA-bit set when computing inter-area routes. In
practice, this fixes interoperability issues with vendors that set
the LA-bit in loopback prefixes (among other cases).

While here, fix a copy-and-paste error where a log message wasn't
showing accurate information about what happened.

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