Minor clean-up in dshash.{c,h}.

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

src/backend/lib/dshash.c

@@ -129,7 +129,7 @@ struct dshash_table
 
 /* How many buckets are there in each partition at a given size? */
 #define BUCKETS_PER_PARTITION(size_log2)		\
-	(UINT64CONST(1) << NUM_SPLITS(size_log2))
+	(((size_t) 1) << NUM_SPLITS(size_log2))
 
 /* Max entries before we need to grow.  Half + quarter = 75% load factor. */
 #define MAX_COUNT_PER_PARTITION(hash_table)				\

src/include/lib/dshash.h

@@ -55,7 +55,7 @@ typedef struct dshash_parameters
 	int			tranche_id;		/* The tranche ID to use for locks */
 } dshash_parameters;
 
-/* Forward declaration of private types for use only by dht.c. */
+/* Forward declaration of private types for use only by dshash.c. */
 struct dshash_table_item;
 typedef struct dshash_table_item dshash_table_item;