command/storage: check if storage is already mounted

Mounting one storage URI twice on different mount points can lead to
conflicts with the database cache file, and it doesn't make a lot of
sense.

But most importantly, our udisks storage plugin will unmount the disk
from the kernel VFS, and if two exist, they will compete with each
others.  We could (and should) fix this in the udisks storage plugin,
but for now, this workaround is good enough (and useful).

src/command/StorageCommands.cxx

@@ -203,6 +203,11 @@ handle_mount(Client &client, Request args, Response &r)
 		return CommandResult::ERROR;
 	}
 
+	if (composite.IsMounted(remote_uri)) {
+		r.Error(ACK_ERROR_ARG, "This storage is already mounted");
+		return CommandResult::ERROR;
+	}
+
 	auto &event_loop = instance.io_thread.GetEventLoop();
 	auto storage = CreateStorageURI(event_loop, remote_uri);
 	if (storage == nullptr) {