flow_dissector: use RCU protection to fetch dev_net()

flow_dissector: use RCU protection to fetch dev_net()

__skb_flow_dissect() can be called from arbitrary contexts.

It must extend its RCU protection section to include
the call to dev_net(), which can become dev_net_rcu().

This makes sure the net structure can not disappear under us.

Fixes: 9b52e3f267a6 ("flow_dissector: handle no-skb use case")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250205155120.1676781-10-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>