staging: erofs: avoid magic constants when initializing clusterbits

Currently erofs only supports clustersize == blocksize.
and clustersize == 2^n * blocksize will be supported in the future.
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/erofs/super.c

@@ -116,9 +116,10 @@ static int superblock_read(struct super_block *sb)
 #endif
 	sbi->islotbits = ffs(sizeof(struct erofs_inode_v1)) - 1;
 #ifdef CONFIG_EROFS_FS_ZIP
-	sbi->clusterbits = 12;
+	/* TODO: clusterbits should be related to inode */
+	sbi->clusterbits = blkszbits;
 
-	if (1 << (sbi->clusterbits - 12) > Z_EROFS_CLUSTER_MAX_PAGES)
+	if (1 << (sbi->clusterbits - PAGE_SHIFT) > Z_EROFS_CLUSTER_MAX_PAGES)
 		errln("clusterbits %u is not supported on this kernel",
 			sbi->clusterbits);
 #endif