CVE-2025-38097: espintcp: remove encap socket caching to avoid reference leak

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

espintcp: remove encap socket caching to avoid reference leak

The current scheme for caching the encap socket can lead to reference leaks when we try to delete the netns.

The reference chain is: xfrmstate -> enacpsk -> netns

Since the encap socket is a userspace socket, it holds a reference on the netns. If we delete the espintcp state (through flush or individual delete) before removing the netns, the reference on the socket is dropped and the netns is correctly deleted. Otherwise, the netns may not be reachable anymore (if all processes within the ns have terminated), so we cannot delete the xfrm state to drop its reference on the socket.

This patch results in a small (~2% in my tests) performance regression.

A GC-type mechanism could be added for the socket cache, to clear references if the state hasn't been used "recently", but it's a lot more complex than just not caching the socket.