ipv6: compare sernum when walking fib for /proc/net/ipv6_route as safety net

ipv6: compare sernum when walking fib for /proc/net/ipv6_route as safety net

This patch provides an additional safety net against NULL
pointer dereferences while walking the fib trie for the new
/proc/net/ipv6_route walkers. I never needed it myself and am unsure
if it is needed at all, but the same checks where introduced in
2bec5a369ee79576a3eea2c23863325089785a2c ("ipv6: fib: fix crash when
changing large fib while dumping it") to fix NULL pointer bugs.

This patch is separated from the first patch to make it easier to revert
if we are sure we can drop this logic.

Cc: Ben Greear <greearb@candelatech.com>
Cc: Patrick McHardy <kaber@trash.net>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>