pgsql: Make min_parallel_relation_size's default value platform-indepen

Make min_parallel_relation_size's default value platform-independent.

The documentation states that the default value is 8MB, but this was
only true at BLCKSZ = 8kB, because the default was hard-coded as 1024.
Make the code match the docs by computing the default as 8MB/BLCKSZ.

Oversight in commit 75be66464, noted pursuant to a gripe from Peter E.

Discussion: <90634e20-097a-e4fd-67d5-fb2c42f0dd71@2ndquadrant.com>

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/72ce78162c6c4e52b7a9bb1a2de50daee0a3628e

Modified Files
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src/backend/utils/misc/guc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)