todo: upstream our ioctls to rustix, forbid unsafe

[allisonkarlitskaya]

We have a small number of custom ioctl implementations:

• FS_IOC_ENABLE_VERITY
• FS_IOC_MEASURE_VERITY
• LOOP_CTL_GET_FREE
• LOOP_CONFIGURE

these are all of the unsafe code in all of composefs-rs.

The reason we implement those ourselves is pretty simple: rustix doesn't contain support. We should look into seeing if upstream would accept these so that we can forbid unsafe code in composefs-rs.