pgsql: Fix brin index summarizing while vacuuming.

Fix brin index summarizing while vacuuming.

If the number of heap blocks is not multiples of pages per range, the
summarizing produces wrong summary information for the last brin index
tuple while vacuuming.

Problem reported by Tatsuo Ishii and fixed by Amit Langote.

Discussion at "[HACKERS] BRIN INDEX value (message id :20150903.174935.1946402199422994347.t-ishii@sraoss.co.jp)
Backpatched to 9.5 in which brin index was added.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b1cbc8529d076b71c9c2f8403139f7235072a357

Modified Files
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src/backend/access/brin/brin.c |    9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)