Properly access a buffer's LSN using existing access macros instead of abusing knowledge of page layout.
Properly access a buffer's LSN using existing access macros instead of abusing
knowledge of page layout.

Stolen from Jonah Harris' CRC patch

Chibin/gpdb,edespino/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,Chibin/gpdb,greenplum-db/gpdb,edespino/gpdb,ashwinstar/gpdb,pavanvd/postgres-xl,zeroae/postgres-xl,yuanzhao/gpdb,xinzweb/gpdb,oberstet/postgres-xl,postmind-net/postgres-xl,postmind-net/postgres-xl,snaga/postgres-xl,Chibin/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,xinzweb/gpdb,yuanzhao/gpdb,xinzweb/gpdb,lisakowen/gpdb,ashwinstar/gpdb,Chibin/gpdb,yuanzhao/gpdb,lisakowen/gpdb,ashwinstar/gpdb,tpostgres-projects/tPostgres,kmjungersen/PostgresXL,Chibin/gpdb,greenplum-db/gpdb,zeroae/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,yuanzhao/gpdb,edespino/gpdb,kmjungersen/PostgresXL,Postgres-XL/Postgres-XL,ovr/postgres-xl,lisakowen/gpdb,tpostgres-projects/tPostgres,xinzweb/gpdb,Postgres-XL/Postgres-XL,Chibin/gpdb,pavanvd/postgres-xl,kmjungersen/PostgresXL,50wu/gpdb,jmcatamney/gpdb,techdragon/Postgres-XL,xinzweb/gpdb,50wu/gpdb,postmind-net/postgres-xl,snaga/postgres-xl,ovr/postgres-xl,zeroae/postgres-xl,arcivanov/postgres-xl,xinzweb/gpdb,greenplum-db/gpdb,ovr/postgres-xl,lisakowen/gpdb,50wu/gpdb,pavanvd/postgres-xl,edespino/gpdb,oberstet/postgres-xl,zeroae/postgres-xl,greenplum-db/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,yazun/postgres-xl,yazun/postgres-xl,jmcatamney/gpdb,Chibin/gpdb,Postgres-XL/Postgres-XL,Chibin/gpdb,50wu/gpdb,yazun/postgres-xl,zeroae/postgres-xl,jmcatamney/gpdb,ovr/postgres-xl,techdragon/Postgres-XL,edespino/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,arcivanov/postgres-xl,yazun/postgres-xl,postmind-net/postgres-xl,edespino/gpdb,50wu/gpdb,arcivanov/postgres-xl,tpostgres-projects/tPostgres,arcivanov/postgres-xl,tpostgres-projects/tPostgres,edespino/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,yuanzhao/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,snaga/postgres-xl,Postgres-XL/Postgres-XL,adam8157/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,edespino/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,arcivanov/postgres-xl,adam8157/gpdb,ovr/postgres-xl,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,kmjungersen/PostgresXL,ashwinstar/gpdb,yuanzhao/gpdb,kmjungersen/PostgresXL,arcivanov/postgres-xl,ashwinstar/gpdb,edespino/gpdb,xinzweb/gpdb,50wu/gpdb,snaga/postgres-xl,edespino/gpdb,tpostgres-projects/tPostgres,yazun/postgres-xl,oberstet/postgres-xl,50wu/gpdb,snaga/postgres-xl,Chibin/gpdb,Chibin/gpdb,ashwinstar/gpdb,postmind-net/postgres-xl,oberstet/postgres-xl,techdragon/Postgres-XL,pavanvd/postgres-xl