CVE-2026-68170: mptcp: fix stale skb->sk reference on subflow close

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

mptcp: fix stale skb->sk reference on subflow close

The backlog list is updated by mptcpdataready() under mptcpdatalock(). The cleanup of backlog references to a closing subflow, however, was performed in mptcpclosessk(), before mptcpclosessk() acquires the ssk lock, and while holding neither the ssk lock nor mptcpdatalock().

Because that traversal ran without mptcpdatalock(), concurrent softirq RX processing on another CPU (subflowdataready() -> mptcpdataready() -> mptcpaddbacklog(), under mptcpdatalock()) could add a backlog entry referencing the ssk while the cleanup loop was in progress. Such an entry could be missed by the cleanup, or the concurrent list update could corrupt the traversal, leaving skb->sk pointing at the ssk after it is freed.

A later mptcpbacklogpurge() then dereferences the stale pointer, triggering a warning in inetsockdestruct() (ssk->skrmemalloc != 0) followed by a use-after-free in mptcpbacklogpurge().

Fix this by moving the backlog cleanup into mptcpclosessk(), after subflow->closing is set to 1 and while the ssk lock is still held, serialized under mptcpdatalock(). The cleanup runs only on the push path (MPTCPCFPUSH), where backlog references accumulate; on other teardown paths the caller already handles cleanup.

With subflow->closing set and mptcpdatalock() held across the purge, any concurrent mptcpdataready() either completes its enqueue before the purge runs and is caught, or observes closing=1 and bails out. Once mptcpdataunlock() is reached, no new skb referencing the ssk can be enqueued, so the cleanup is exhaustive.

Remove the unprotected traversal from mptcpclosessk() entirely.