You are asking it to listen on your local port 22 and forward connections to a remote system's port 8090. You can't do that, because your local port 22 is already taken by your local SSH server.
I think what you are looking for is remote forwarding. Replacing
-L 22:localhost:8090 with
-R 8090:localhost:22 will tell the remote host to listen on port 8090 and forward requests to your SSH server.
If you are leaving the connection running so you can get in later from a remote site, then you are going to want to make sure the connection doesn't time-out due to inactivity by adding the relevant options (
-o TCPKeepAlive=yes or
So you'll end up with something like:
ssh -N user@my_server -R 8090:localhost:22 -o ServerAliveInterval=30
Also, if one of the network hops between you and the server is down at any point, the connection will drop despite any KeepAlive options you specify, so you might want to add this command to inittab, or look into the daemontools package or your distro's equivalent , so that it always starts on boot and is restarted when it exits for some reason other then system shutdown (or you could run it from a shell script that loops infinitely, but init or daemontools are cleaner solutions).