[IPV6] ROUTE: Make sending algorithm more friendly with RFC 4861.

We omit (or delay) sending NSes for known-to-unreachable routers (in
NUD_FAILED state) according to RFC 4191 (Default Router Preferences
and More-Specific Routes).  But this is not fully compatible with RFC
4861 (Neighbor Discovery Protocol for IPv6), which does not remember
unreachability of neighbors.

So, let's avoid mixing sending algorithm of RFC 4191 and that of RFC
4861, and make the algorithm more friendly with RFC 4861 if RFC 4191
is disabled.

Issue was found by IPv6 Ready Logo Core Self_Test 1.5.0b2 (by TAHI
Project), and has been tracked down by Mitsuru Chinen
<mitch@linux.vnet.ibm.com>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>