rxrpc: Allow the kernel to mark a call as being non-interruptible

Allow kernel services using AF_RXRPC to indicate that a call should be
non-interruptible.  This allows kafs to make things like lock-extension and
writeback data storage calls non-interruptible.

If this is set, signals will be ignored for operations on that call where
possible - such as waiting to get a call channel on an rxrpc connection.

It doesn't prevent UDP sendmsg from being interrupted, but that will be
handled by packet retransmission.

rxrpc_kernel_recv_data() isn't affected by this since that never waits,
preferring instead to return -EAGAIN and leave the waiting to the caller.

Userspace initiated calls can't be set to be uninterruptible at this time.

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

net/rxrpc/call_object.c

@@ -241,6 +241,8 @@ struct rxrpc_call *rxrpc_new_client_call(struct rxrpc_sock *rx,
 		return call;
 	}
 
+	if (p->intr)
+		__set_bit(RXRPC_CALL_IS_INTR, &call->flags);
 	call->tx_total_len = p->tx_total_len;
 	trace_rxrpc_call(call, rxrpc_call_new_client, atomic_read(&call->usage),
 			 here, (const void *)p->user_call_ID);