DropRelFileNodeBuffers failed to fix the state of the lookup hash table

DropRelFileNodeBuffers failed to fix the state of the lookup hash table
that was added to localbuf.c in 8.1; therefore, applying it to a temp table
left corrupt lookup state in memory.  The only case where this had a
significant chance of causing problems was an ON COMMIT DELETE ROWS temp
table; the other possible paths left bogus state that was unlikely to
be used again.  Per report from Csaba Nagy.