Fix "failed to re-find parent key" btree VACUUM failure by tweaking

Fix "failed to re-find parent key" btree VACUUM failure by tweaking
_bt_pagedel to recover from the failure: just search the whole parent level
if searching to the right fails.  This does nothing for the underlying problem
that index keys became out-of-order in the grandparent level.  However, we
believe that there is no other consequence worse than slightly inefficient
searching, so this narrow patch seems like the safest solution for the back
branches.