ipv6: Rename rt6_nh.next to rt6_nh.list.

ipv6: Rename rt6_nh.next to rt6_nh.list.

ip6_route_multipath_add() allocates struct rt6_nh for each config
of multipath routes to link them to a local list rt6_nh_list.

struct rt6_nh.next is the list node of each config, so the name
is quite misleading.

Let's rename it to list.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/netdev/c9bee472-c94e-4878-8cc2-1512b2c54db5@redhat.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418000443.43734-11-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>