Sounds like you've taken a wrong turn somewhere.
First off, look at what the acronyms stand for - NAS = Network Attached Storage. The whole point of a NAS is that the volumes are accessed over a network - typically ethernet. Therefore if you're building a NAS you only need an ethernet network between the server and the client(s). You've spent additional money on a fiber channel switch and cards that you don't need.
Fiber channel is typically used in a SAN - Storage Area Network. This is where clients communicate with the SAN server over a dedicated storage link. Off hand I have no idea whether your Linux server can be configured as a SAN host - it usually takes a specific SAN controller to do what you describe. There may be software solutions around that will do what you want but I'm not aware of them