osd: use (robust) helper for setting exists or clearing whiteout

osd: use (robust) helper for setting exists or clearing whiteout

The current blanket check in prepare_transaction() will trigger only when
there is a net obs.exists change from the commited obs to new_obs.
However, this misses the case where the first osd_op is a delete and then a
subsequent osd_op recreates the object.  Changing the whiteout check to
look only at new_obs does not work because it fails to understand when
_delete_oid sets the whiteout and will simply clear it again.

In order to support sequences of delete + create in general, we need to do
the whiteout flag clearing when the actual create happens (to match the
fact that we set it when we process the delete osd_op).  Use a helper to
do this and consolidate most other obs.exists = true code to use it.

Backport: giant, firefly
Signed-off-by: Sage Weil <sage@redhat.com>