Fix GIN data page split ratio calculation.
Fix GIN data page split ratio calculation.

The code that tried to split a page at 75/25 ratio, when appending to the
end of an index, was buggy in two ways. First, there was a silly typo that
caused it to just fill the left page as full as possible. But the logic as
it was intended wasn't correct either, and would actually have given a ratio
closer to 60/40 than 75/25.

Gaetano Mendola spotted the typo. Backpatch to 9.4, where this code was added.

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