Linux: ovpn: replace bind when clearing stale local source

In the Linux kernel, the following vulnerability has been resolved:

ovpn: replace bind when clearing stale local source

The UDP output fallback clears bind->local in place when the remembered source address is no longer usable. The bind is RCU-published and read locklessly by concurrent TX, so an IPv6 reader can observe a torn address.

Retry the route lookup with source address autoselection without modifying the bind. After a successful lookup, revalidate the bind and route key under peer->lock, reset the dst cache, and best-effort publish a replacement bind with a wildcard local address.

Do not cache the resolved dst when clearing the local source. Replacing the source invalidates all per-CPU cache entries, while dst_cache_set_ip4 and dst_cache_set_ip6 update only the current CPU slot. The current packet can still use the resolved route; if bind allocation fails, a later cache miss retries the repair.