ipv6: introduce a hash table to store dst cache

Add a hash table into struct rt6_info in order to store dst caches
created by pmtu discovery and ip redirect in ipv6 routing code.
APIs to add dst cache, delete dst cache, find dst cache and update
dst cache in the hash table are implemented and will be used in later
commits.
This is a preparation work to move all cache routes into the exception
table instead of getting inserted into the fib6 tree.

Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>