Fix wrong keysize in PrivateRefCountHash creation.

In 4b4b680c3 I accidentally used sizeof(PrivateRefCountArray) instead of
sizeof(PrivateRefCountEntry) when creating the refcount overflow
hashtable. As the former is bigger than the latter, this luckily only
resulted in a slightly increased memory usage when many buffers are
pinned in a backend.

Reported-By: Takashi Horikawa
Discussion: 73FA3881462C614096F815F75628AFCD035A48C3@BPXM01GP.gisp.nec.co.jp
Backpatch: 9.5, where thew new ref count infrastructure was introduced

src/backend/storage/buffer/bufmgr.c

@@ -2103,7 +2103,7 @@ InitBufferPoolAccess(void)
 
 	MemSet(&hash_ctl, 0, sizeof(hash_ctl));
 	hash_ctl.keysize = sizeof(int32);
-	hash_ctl.entrysize = sizeof(PrivateRefCountArray);
+	hash_ctl.entrysize = sizeof(PrivateRefCountEntry);
 
 	PrivateRefCountHash = hash_create("PrivateRefCount", 100, &hash_ctl,
 									  HASH_ELEM | HASH_BLOBS);