pgsql: Fix integer overflow bug in GiST buffering build calculations.

Fix integer overflow bug in GiST buffering build calculations.

The result of (maintenance_work_mem * 1024) / BLCKSZ doesn't fit in a signed
32-bit integer, if maintenance_work_mem >= 2GB. Use double instead. And
while we're at it, write the calculations in an easier to understand form,
with the intermediary steps written out and commented.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/4bc6fb57f774ea18187fd8565aad9994160bfc17

Modified Files
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src/backend/access/gist/gistbuild.c |   32 +++++++++++++++++++++++---------
1 files changed, 23 insertions(+), 9 deletions(-)