For consistency with other code that deals in numbers of buckets, the
macro BUCKETS_PER_PARTITION should produce a value of type size_t.
Also, fix a mention of an obsolete proposed name for dshash.c that
appeared in a comment.

Author: Thomas Munro, based on an observation from Amit Kapila
Discussion: https://postgr.es/m/CAA4eK1%2BBOp5aaW3aHEkg5Bptf8Ga_BkBnmA-%3DXcAXShs0yCiYQ%40mail.gmail.com