filejournal: fix journal write_pos advance
filejournal: fix journal write_pos advance

This was broken by bd4188a02abff9efffb87a0a2031efe51c1b4d9a.  @pos needs to
be advanced (it is pass by reference) or else we just overwrite the same
bytes at the journal start over and over again.

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