ipv6: adopt dst_dev() helper

ipv6: adopt dst_dev() helper

[ Upstream commit 1caf27297215a5241f9bfc9c07336349d9034ee3 ]

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

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250630121934.3399505-9-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[Harshit: Backport to 6.12.y, pulled this is a prerequisite]
Stable-dep-of: 99a2ace61b21 ("net: use dst_dev_rcu() in sk_setup_caps()")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>