fix a bug about version control in function getPartitionDefs.

[li-bupt]

fix a bug about version control in function getPartitionDefs (pg_dump). GPDB6 VERSION (such as 90426) is generally greater than GPDB6_MAJOR_PGVERSION (90400). so we need this function to take effect in GPDB6.

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[vmwclabot]

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[li-bupt]

any progress? :)