#7317 ENOSPC and OSD epoch barrier handling


Related:
https://github.com/ceph/ceph-qa-suite/pull/225
https://github.com/ceph/ceph-client/pull/7
Hmm, I was more thinking about the return values section in http://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html

But it is indeed not clear about what might have changed on the disk if you return an error value, so I think we're good! Hurray.