CVE-2025-21864: tcp: drop secpath at the same time as we currently drop dst

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

tcp: drop secpath at the same time as we currently drop dst

Xiumei reported hitting the WARN in xfrm6tunnelnetexit while running tests that boil down to: - create a pair of netns - run a basic TCP test over ipcomp6 - delete the pair of netns

The xfrmstate found on spibyaddr was not deleted at the time we delete the netns, because we still have a reference on it. This lingering reference comes from a secpath (which holds a ref on the xfrmstate), which is still attached to an skb. This skb is not leaked, it ends up on skreceivequeue and then gets defer-free'd by skbattemptdeferfree.

The problem happens when we defer freeing an skb (push it on one CPU's deferlist), and don't flush that list before the netns is deleted. In that case, we still have a reference on the xfrmstate that we don't expect at this point.

We already drop the skb's dst in the TCP receive path when it's no longer needed, so let's also drop the secpath. At this point, tcpfilter has already called into the LSM hooks that may require the secpath, so it should not be needed anymore. However, in some of those places, the MPTCP extension has just been attached to the skb, so we cannot simply drop all extensions.