rxrpc: Fix checker warnings and errors

Fix various issues detected by checker.

Errors:

 (*) rxrpc_discard_prealloc() should be using rcu_assign_pointer to set
     call->socket.

Warnings:

 (*) rxrpc_service_connection_reaper() should be passing NULL rather than 0 to
     trace_rxrpc_conn() as the where argument.

 (*) rxrpc_disconnect_client_call() should get its net pointer via the
     call->conn rather than call->sock to avoid a warning about accessing
     an RCU pointer without protection.

 (*) Proc seq start/stop functions need annotation as they pass locks
     between the functions.

False positives:

 (*) Checker doesn't correctly handle of seq-retry lock context balance in
     rxrpc_find_service_conn_rcu().

 (*) Checker thinks execution may proceed past the BUG() in
     rxrpc_publish_service_conn().

 (*) Variable length array warnings from SKCIPHER_REQUEST_ON_STACK() in
     rxkad.c.

Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/call_object.c

@@ -219,6 +219,7 @@ struct rxrpc_call *rxrpc_new_client_call(struct rxrpc_sock *rx,
 					 gfp_t gfp,
 					 unsigned int debug_id)
 	__releases(&rx->sk.sk_lock.slock)
+	__acquires(&call->user_mutex)
 {
 	struct rxrpc_call *call, *xcall;
 	struct rxrpc_net *rxnet = rxrpc_net(sock_net(&rx->sk));