Dept. of further reflection: I looked around to see if any other callers

of XLogInsert had the same sort of checkpoint interlock problem as
RecordTransactionCommit, and indeed I found some.  Btree index build
and ALTER TABLE SET TABLESPACE write data outside the friendly confines
of the buffer manager, and therefore they have to take their own
responsibility for checkpoint interlock.  The easiest solution seems to
be to force smgrimmedsync at the end of the index build or table copy,
even when the operation is being WAL-logged.  This is sufficient since
the new index or table will be of interest to no one if we don't get
as far as committing the current transaction.