filestore: close btrfs transactions cleanly
filestore: close btrfs transactions cleanly

Don't just close the fd.  We may add a mount option that makes a messy
transaction close wedge the fs instead of committing partial results.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph