ipv6: Only create RTF_CACHE routes after encountering pmtu exception

ipv6: Only create RTF_CACHE routes after encountering pmtu exception

This patch creates a RTF_CACHE routes only after encountering a pmtu
exception.

After ip6_rt_update_pmtu() has inserted the RTF_CACHE route to the fib6
tree, the rt->rt6i_node->fn_sernum is bumped which will fail the
ip6_dst_check() and trigger a relookup.

Signed-off-by: Martin KaFai Lau <[email protected]>
Cc: Hannes Frederic Sowa <[email protected]>
Cc: Steffen Klassert <[email protected]>
Cc: Julian Anastasov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>