While looking at a slow update issue (which I'm guessing is
unpredictable I/O latency in an OpenStack instance), I noticed
in one of the traces we were inside a fsync here.
Dropping the fsync here is just another of a long series of unwinding
them - we `syncfs()` the sysroot fd and `/boot` and we have a big
`sync()` anyways.
Closes: #508
Approved by: jlebon