nexthop: Add netlink handlers for bucket get

Allow getting (but not setting) individual buckets to inspect the next hop
mapped therein, idle time, and flags.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>