Fix ipv6 routing table entry limit.

As discussed in other threads, the routing table lives in the same
data structure as cached routing entries.  This enforces a false
limit on the number of routing table entries one can install
on ipv6 and this is becomming a real problem for people.

Fix this by adding a DST_NOCOUNT flag and use it for ipv6 routing
table entries.

No special handling is necessary when cloning or copying since
the flags are never directly copied by the clone/copy code in
ipv6.

This is a 2 patch series now in order to sanitize the dst->flags
setting done in net/ipv6/route.c

I've tested this by adding and removing ~16K ipv6 routes over
and over again, the routing cache limit never got hit.

Simon, you'll need to work out that crash you were seeing since
I can't reproduce it on any of my machines.