osd/PrimaryLogPG: keep snapset on whiteout head if luminous

osd/PrimaryLogPG: keep snapset on whiteout head if luminous

If REQUIRE_LUMINOUS is set on the OSDMap, put the SnapSet on the head
and make it a whiteout.  This is simpler and will eventually (once all
the old snapdir objects have been fixed up) let us remove a ton of
snapdir-related headaches.

This is surprisingly simple.  Everywhere else we work with snapdir is
already set up to handle the snapset on the head.  The only difference is
that we are preventing outselves from moving from the snapset-on-head
state to the snapset-on-snapdir-with-no-head state.  The only time this
happens is when the object is logically deleted in _delete_oid.

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