pgsql: 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.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/79ee6370ee803b36c2649cfb7655affa2c1007f1

Modified Files
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src/backend/access/gin/gindatapage.c |   29 +++++++++++++++++------------
1 file changed, 17 insertions(+), 12 deletions(-)