SUNRPC: pin upper rpc_clnt across the TLS connect_worker

In the Linux kernel, the following vulnerability has been resolved: SUNRPC: pin upper rpc_clnt across the TLS connect_worker The TLS connect path has a use-after-free: nothing pins the upper rpc_clnt across the delayed connect_worker. xs_connect() stores task->tk_client in sock_xprt::clnt as a raw pointer and queues the worker; for TLS-secured transports that worker is xs_tcp_tls_setup_socket(), which reads several fields out of the saved pointer (cl_timeout, cl_program, cl_prog, cl_vers, cl_cred, cl_stats) to construct the args for the inner handshake rpc_clnt. The xprt does not reference the rpc_clnt; the rpc_clnt references the xprt. xs_destroy() does cancel the connect_worker, but it runs only when the xprt's refcount drops to zero, which cannot happen until the rpc_clnt releases its cl_xprt reference in rpc_free_client_work(). When a TLS handshake fails fatally (for example, an mTLS mount whose client cert does not match the server), the connecting task is woken with -EACCES and exits, the mount caller invokes rpc_shutdown_client(), and the upper rpc_clnt is freed before the queued connect_worker fires. xs_tcp_tls_setup_socket() then dereferences the freed clnt, producing the refcount_t underflow Michael Nemanov reported. Take a reference on the upper rpc_clnt in xs_connect() for TLS transports via a new rpc_hold_client() helper, and drop it in the connect_worker's exit path with rpc_release_client(). The xprt_lock_connect() / xprt_unlock_connect() pairing already serialises xs_connect() with xs_tcp_tls_setup_socket(), so the take and release are balanced one-for-one. The non-TLS connect worker (xs_tcp_setup_socket) never reads sock_xprt::clnt, so leave that path alone and avoid the clnt-holds-xprt-holds-clnt cycle that would otherwise prevent xprt destruction.