SMB traffic (File server) forwarding

You just need a simple Virtual Server (Name, IP Address, TCP Port (SMB - 445 in your case), and your TCP Profiles).

You can add a default Pool if you like, but it will impact the way you write your iRule.

I would suggest Data Groups for your individual Client Subnets so they are easier to manage (or you can cook it all into your iRule). Just remember to use the "when CLIENT_ACCEPTED" event.

There are lots of examples on DevCentral for "Source IP Bases Routing", so you have lots to choose from and then use the iRules Wiki to get additional details on the commands that they contain to construct your iRule and understand what each portion of the examples you find are doing.