sunrpc: pin svc_xprt across the asynchronous TLS handshake callback

In the Linux kernel, the following vulnerability has been resolved: sunrpc: pin svc_xprt across the asynchronous TLS handshake callback svc_tcp_handshake() stores the raw svc_xprt pointer in tls_handshake_args.ta_data and submits the request through tls_server_hello_x509(). The handshake core takes only sock_hold(req->hr_sk); nothing references the embedding struct svc_sock that svc_tcp_handshake_done() reaches via container_of(). Two close races leave the in-flight callback writing through a freed svc_sock. svc_sock_free() calls tls_handshake_cancel() and discards its return value: a false return means handshake_complete() has already set HANDSHAKE_F_REQ_COMPLETED but hp_done() may not have finished, yet svc_sock_free() proceeds to kfree(svsk). The cancel-loser fall-through inside svc_tcp_handshake() itself produces the same window: when wait_for_completion_interruptible_timeout() returns <= 0 (timeout or signal) and tls_handshake_cancel() returns false, the function does not drain, returns, and svc_handle_xprt() calls svc_xprt_received(), which clears XPT_BUSY and can drop the last reference. A concurrent close then runs svc_sock_free() while svc_tcp_handshake_done() is still updating xpt_flags and walking svsk->sk_handshake_done. The corruption surfaces as set_bit/clear_bit RMW into the freed xpt_flags slab slot and as complete_all() walking and writing the freed wait_queue_head_t list embedded in sk_handshake_done -- a slab-corruption primitive, not a benign read. The path is reachable on any TLS-enabled NFS server whenever a connection close overlaps the tlshd downcall delivery window; the interruptible wait means signal delivery suffices, not just SVC_HANDSHAKE_TO expiry. Take svc_xprt_get(xprt) immediately before tls_server_hello_x509() so the in-flight callback owns its own reference. Release it on the two edges where the callback is guaranteed not to fire -- submission failure from tls_server_hello_x509() and a successful tls_handshake_cancel() -- and at the tail of svc_tcp_handshake_done() after complete_all(). [cel: rewrote commit message to describe the actual change]