erofs: clear compacted_2b if compacted_4b_initial > totalidx

Currently, the whole indexes will only be compacted 4B if
compacted_4b_initial > totalidx. So, the calculated compacted_2b
is worthless for that case. It may waste CPU resources.

No need to update compacted_4b_initial as mkfs since it's used to
fulfill the alignment of the 1st compacted_2b pack and would handle
the case above.

We also need to clarify compacted_4b_end here. It's used for the
last lclusters which aren't fitted in the previous compacted_2b
packs.

Some messages are from Xiang.

Link: https://lore.kernel.org/r/20210914035915.1190-1-zbestahu@gmail.comSigned-off-by: Yue Hu <huyue2@yulong.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
[ Gao Xiang: it's enough to use "compacted_4b_initial < totalidx". ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/zmap.c

@@ -369,7 +369,8 @@ static int compacted_load_cluster_from_disk(struct z_erofs_maprecorder *m,
 	if (compacted_4b_initial == 32 / 4)
 		compacted_4b_initial = 0;
 
-	if (vi->z_advise & Z_EROFS_ADVISE_COMPACTED_2B)
+	if ((vi->z_advise & Z_EROFS_ADVISE_COMPACTED_2B) &&
+	    compacted_4b_initial < totalidx)
 		compacted_2b = rounddown(totalidx - compacted_4b_initial, 16);
 	else
 		compacted_2b = 0;