Fix gin_desc routine to match the WAL format.
Fix gin_desc routine to match the WAL format.

In the GIN incomplete-splits patch, I used BlockIdDatas to store the block
number of left and right children, when inserting a downlink after a split
to an internal page posting list page. But gin_desc thought they were stored
as BlockNumbers.

oberstet/postgres-xl,xinzweb/gpdb,techdragon/Postgres-XL,ovr/postgres-xl,50wu/gpdb,jmcatamney/gpdb,oberstet/postgres-xl,ovr/postgres-xl,xinzweb/gpdb,adam8157/gpdb,oberstet/postgres-xl,Postgres-XL/Postgres-XL,jmcatamney/gpdb,Postgres-XL/Postgres-XL,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,50wu/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,50wu/gpdb,lisakowen/gpdb,ashwinstar/gpdb,pavanvd/postgres-xl,jmcatamney/gpdb,zeroae/postgres-xl,adam8157/gpdb,ovr/postgres-xl,xinzweb/gpdb,Postgres-XL/Postgres-XL,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,techdragon/Postgres-XL,xinzweb/gpdb,50wu/gpdb,jmcatamney/gpdb,zeroae/postgres-xl,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,yazun/postgres-xl,Postgres-XL/Postgres-XL,ashwinstar/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,yazun/postgres-xl,zeroae/postgres-xl,pavanvd/postgres-xl,pavanvd/postgres-xl,pavanvd/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,techdragon/Postgres-XL,yazun/postgres-xl,ashwinstar/gpdb,50wu/gpdb,ashwinstar/gpdb,adam8157/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,xinzweb/gpdb,zeroae/postgres-xl,adam8157/gpdb,pavanvd/postgres-xl,ovr/postgres-xl,techdragon/Postgres-XL,xinzweb/gpdb,yazun/postgres-xl,oberstet/postgres-xl,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,yazun/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,jmcatamney/gpdb,ovr/postgres-xl,zeroae/postgres-xl,50wu/gpdb,techdragon/Postgres-XL