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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master

Details
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http://git.postgresql.org/pg/commitdiff/c39f5674dfc77802b4245dd92cb4b1e35499a5a5

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