Remove unnecessary variables in _hash_splitbucket().
Remove unnecessary variables in _hash_splitbucket().

Commit ed9cc2b5df59fdbc50cce37399e26b03ab2c1686 made it unnecessary to pass
start_nblkno to _hash_splitbucket(), and for that matter unnecessary to
have the internal nblkno variable either.  My compiler didn't complain
about that, but some did.  I also rearranged the use of oblkno a bit to
make that case more parallel.

Report and initial patch by Petr Jelinek, rearranged a bit by me.
Back-patch to all branches, like the previous patch.

50wu/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,xinzweb/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,lisakowen/gpdb,ashwinstar/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,adam8157/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,xinzweb/gpdb,jmcatamney/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb