f3read: tighten assert()

The last assert() in iterate_files() was doing an integer division
before comparing values, so there was potential for a bug going
silent due to the truncation fo the integer division.
This patch rewrite the test with a multiplication.

f3read.c

@@ -382,7 +382,7 @@ static void iterate_files(const char *path, const long *files,
 		files++;
 	}
 	assert(!gettimeofday(&t2, NULL));
-	assert(tot_size / SECTOR_SIZE ==
+	assert(tot_size == SECTOR_SIZE *
 		(tot_ok + tot_corrupted + tot_changed + tot_overwritten));
 
 	/* Notice that not reporting `missing' files after the last file