Circular-freelist bug is still there

I just saw the parallel regression tests hang up again.  Inspection
revealed that StrategyInvalidateBuffer() was stuck in an infinite loop
because the freelist was circular.

(gdb) p StrategyControl->listFreeBuffers
$5 = 579
(gdb) p BufferDescriptors[579]
$6 = {bufNext = 106, data = 4991904, tag = {rnode = {tblNode = 17142,     relNode = 143947}, blockNum = 0}, buf_id =
579,flags = 14, refcount = 0, io_in_progress_lock = 1179, cntx_lock = 1180, cntxDirty = 0 '\000', wait_backend_id = 0}
 
(gdb) p BufferDescriptors[106]
$7 = {bufNext = 684, data = 1117088, tag = {rnode = {tblNode = 17142,     relNode = 143989}, blockNum = 0}, buf_id =
106,flags = 14, refcount = 0, io_in_progress_lock = 233, cntx_lock = 234, cntxDirty = 0 '\000', wait_backend_id = 0}
 
(gdb) p BufferDescriptors[684]
$8 = {bufNext = 579, data = 5852064, tag = {rnode = {tblNode = 17142,     relNode = 143929}, blockNum = 0}, buf_id =
684,flags = 14, refcount = 0, io_in_progress_lock = 1389, cntx_lock = 1390, cntxDirty = 0 '\000', wait_backend_id = 0}
 
(gdb)

Don't have time to chase it right now, but you should know that there's
still a low-probability bug in there.
        regards, tom lane