CVE-2026-74436: rxrpc: serialize kernel accept preallocation with socket teardown

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

rxrpc: serialize kernel accept preallocation with socket teardown

rxrpckernelchargeaccept() reads rx->backlog without any socket/backlog synchronization and passes that raw pointer into rxrpcservicepreallocone(). A concurrent rxrpcdiscardprealloc() sets rx->backlog = NULL and frees the backlog rings, so a kernel preallocation worker can keep using a freed struct rxrpcbacklog while updating backloghead/tail and array slots.

Serialize the state check and backlog lookup with the socket lock, and reject kernel preallocation once teardown has disabled listening or discarded the service backlog.