crush/CrushWrapper: fix create_or_move_item when name exists but item does not

crush/CrushWrapper: fix create_or_move_item when name exists but item does not

We were using item_exists(), which simply checks if we have a name defined
for the item.  Instead, use _search_item_exists(), which looks for an
instance of the item somewhere in the hierarchy.  This matches what
get_item_weightf() is doing, which ensures we get a non-negative weight
that converts properly to floating point.

Backport: giant, firefly
Fixes: #9998
Reported-by: Pawel Sadowski <ceph@sadziu.pl>
Signed-off-by: Sage Weil <sage@redhat.com>