CVE-2026-74262: kcm: use WRITE_ONCE() when changing lower socket callbacks

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

kcm: use WRITEONCE() when changing lower socket callbacks

kcmattach() replaces a live lower TCP socket's skdataready and skwritespace callbacks with KCM handlers, and kcmunattach() restores them later. Those callback-pointer updates are still plain stores even though the same fields can be read and invoked concurrently on other CPUs.

If another CPU observes an older callback snapshot after the live field has already been restored, callback execution can run with a mismatched target and skuserdata state, leading to stale or misdirected wakeups.

Use WRITEONCE() for the callback replacement and restore operations so these shared callback fields follow the same visibility contract already established by the earlier 4022 fixes.