ospf6d: check N-bit set in Hello packet

ospf6d: check N-bit set in Hello packet

RFC 3101 states both E-bit and N-bit need to be checked when receiving a Hello packet.

"To support the NSSA option an additional check must be made in the function
that handles the receiving of the Hello packet to verify that both the N-bit
and the E-bit found in the Hello packet's option field match the area type and
ExternalRoutingCapability of the area of the receiving interface."

This PR adds the check for the N-bit

Signed-off-by: ckishimo <carles.kishimoto@gmail.com>