ipv6: adopt skb_dst_dev() and skb_dst_dev_net[_rcu]() helpers

Use the new helpers as a step to deal with potential dst->dev races.

v2: fix typo in ipv6_rthdr_rcv() (kernel test robot <lkp@intel.com>)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250630121934.3399505-10-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>