Fix rewrite dissect.exfat

[DissectBot]

It implements access of the file system in a bit weird way. It does not provide get() or iterdir()/scandir() functionality like the other fs implementations. It also does not provide a simple form of the filesystem entry.

Furthermore it fails to parse the root directory from the correct offset.

[Lekensteyn]

Furthermore it fails to parse the root directory from the correct offset.

Could you clarify this claim? I found an issue in ExFAT parsing that should be addressed by #31, perhaps that is the same issue?

[Schamper]

This ticket was publicised from a much much older internal ticket, so I'm afraid that specific claim has bitrotted in human memory 😅. But it's very likely that your fix addresses that specific claim.