#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
I think we want to mark the Inode as failed_flush_on_full or something. The _release_fh() caller (which is used by close(), among others) doesn't pass in a Context, and we want those functions to return ENOSPC if we know that's what will happen.