pfcp: Store struct sock in struct pfcp_dev.

pfcp: Store struct sock in struct pfcp_dev.

pfcp does not need to access struct socket itself in the fast
path; it only reads struct sock, and struct socket is only used
for tunnel setup and teardown.

Let's store struct sock directly in struct pfcp_dev.

pfcp_del_sock() is called from dev->netdev_ops->ndo_uninit().
The 2nd synchronize_net() in unregister_netdevice_many_notify()
ensures that inflight pfcp RX fast paths finish before pfcp_dev
is freed.

Note that synchronize_rcu() is added in the error path of
pfcp_newlink() since free_netdev() will free pfcp_dev immediately
once we remove synchronize_rcu() in udp_tunnel_sock_release().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260502031401.3557229-14-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>