pgsql: Fix brin_summarize_new_values() to check index type and ownershi

Fix brin_summarize_new_values() to check index type and ownership.

brin_summarize_new_values() did not check that the passed OID was for
an index at all, much less that it was a BRIN index, and would fail in
obscure ways if it wasn't (possibly damaging data first?). It also
lacked any permissions test; by analogy to VACUUM, we should only allow
the table's owner to summarize.

Noted by Jeff Janes, fix by Michael Paquier and me

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/e10838026b373f01d1de0f4f7ea80a60c30565da

Modified Files
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src/backend/access/brin/brin.c | 42 +++++++++++++++++++++++++++++++++---
src/test/regress/expected/brin.out | 11 ++++++++++
src/test/regress/sql/brin.sql | 5 +++++
3 files changed, 55 insertions(+), 3 deletions(-)