osdc/ObjectCacher: only call flush callback if we transitions to clean

osdc/ObjectCacher: only call flush callback if we transitions to clean

If we race with e.g. truncate and are in bh_write_commit but the oset
is already clean, we should not call the flush callback (again).

This is reproduced by:

 - kludging slow osd replies into the code (e.g., 2 second delay)
 - mount ceph-fuse with --client-oc-max-dirty-age 1
 - dd if=/dev/zero of=mnt/foo count=1
   sleep 1
   truncate --size 0 mnt/foo
 -> crash

Signed-off-by: Sage Weil <sage@inktank.com>