net: dst: annotate data-races around dst->output

dst_dev_put() can overwrite dst->output while other
cpus might read this field (for instance from dst_output())

Add READ_ONCE()/WRITE_ONCE() annotations to suppress
potential issues.

We will likely need RCU protection in the future.

Fixes: 4a6ce2b6 ("net: introduce a new function dst_dev_put()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250630121934.3399505-6-edumazet@google.com


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