The published versions of the IOSCSIArchitectureModelFamily source code are here. The latest public version is quite old, so it's probably diverged somewhat since then, but should still be somewhat useful - the file you're after is this one.
Apple have been publishing progressively less kernel source code over the years, unfortunately.
Google's site:
operator is extremely helpful for finding source code on Apple's site. For example, this search locates the file you're after. It usually won't find the newest version, but the directories are browseable, so you just need to truncate the URL to the package base, e.g. from the first result in the above search:
http://opensource.apple.com/source/IOSCSIArchitectureModelFamily/IOSCSIArchitectureModelFamily-1.1_35/IOSCSIBlockCommands/IOSCSIBlockCommandsDevice.cpp