Fix more portability issues with casts to Size when using off_t

This should tame the beast, as there are no other places where off_t is
used in the new error messages.

Reported again by longfin, which complained about walsender.c while I
spotted the other two ones while double-checking.

src/backend/replication/slot.c

@@ -1422,7 +1422,7 @@ RestoreSlotFromDisk(const char *name)
 			ereport(PANIC,
 					(errmsg("could not read file \"%s\": read %d of %zu",
 							path, readBytes,
-							ReplicationSlotOnDiskConstantSize)));
+							(Size) ReplicationSlotOnDiskConstantSize)));
 	}
 
 	/* verify magic */