Code review comment for lp:~jameinel/bzr/2.4-fdatasync-ENOTSUP-1075108

I have a bad feeling about ignoring errors there, the bug you're fixing is the first occurrence of fdatasync() failing in a very specific context. I'd rather trap only that specific case and keep failing for the others rather than silently ignoring them (the probability that someone notices the mutter() messages in the log is pretty much zero).