util/set: Do a full search when adding new items

util/set: Do a full search when adding new items

Previously, the set_insert function would bail early if it found a deleted
slot that it could re-use.  However, this is a problem if the key being
inserted is already in the set but further down the list.  If this happens,
the element ends up getting inserted in the set twice.  This commit makes
it so that we walk over all of the possible entries for the given key and
then, if we don't find the key, place it in the available free entry we
found.

Reviewed-by: Eric Anholt <eric@anholt.net>