Add support for the D-port (diagnostic port) fabric switch feature.

(Fabric Switch initiates loopback style port testing)
Signed-off-by: NJoe Carnuccio <joe.carnuccio@qlogic.com>
Signed-off-by: NSaurav Kashyap <saurav.kashyap@qlogic.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>