Only call recurse() when path points to a directory in du(1)

This improves performance by ~30%.

du.c

@@ -47,6 +47,7 @@ du(const char *path, int depth, void *total)
 		return;
 	}
 
+	if (S_ISDIR(st.st_mode))
 		recurse(path, du, depth, &subtotal);
 	*((size_t *)total) += subtotal + nblks(st.st_blocks);