Coroutine benchmark fails to build

The benchmark at

fails to build as

error[E0308]: mismatched types
  --> /home/nordlow/.cargo/registry/src/
63 |     let ret = unsafe { libc::posix_fallocate(file.as_raw_fd(), 0, len as off_t) };
   |                                                                   ^^^^^^^^^^^^ expected i64, found u64

error: aborting due to previous error

error: Could not compile `fs2`.
warning: build failed, waiting for other jobs to finish...
error: build failed

The error occurs in another module.

I'm using Rust version 1.21.

What to do?

It looks like incompatibility between fs2 and libc. Let fs2 author know about this.