ipv6: strengthen fallback fragmentation id generation

ipv6: strengthen fallback fragmentation id generation

First off, we don't need to check for non-NULL rt any more, as we are
guaranteed to always get a valid rt6_info. Drop the check.

In case we couldn't allocate an inet_peer for fragmentation information
we currently generate strictly incrementing fragmentation ids for all
destination. This is done to maximize the cycle and avoid collisions.

Those fragmentation ids are very predictable. At least we should try to
mix in the destination address.

While it should make no difference to simply use a PRNG at this point,
secure_ipv6_id ensures that we don't leak information from prandom,
so its internal state could be recoverable.

This fallback function should normally not get used thus this should
not affect performance at all. It is just meant as a safety net.

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>