erofs: fix compact 4B support for 16k block size

In compact 4B, two adjacent lclusters are packed together as a unit to
form on-disk indexes for effective random access, as below:

(amortized = 4, vcnt = 2)
       _____________________________________________
      |___@_____ encoded bits __________|_ blkaddr _|
      0        .                                    amortized * vcnt = 8
      .             .
      .                  .              amortized * vcnt - 4 = 4
      .                        .
      .____________________________.
      |_type (2 bits)_|_clusterofs_|

Therefore, encoded bits for each pack are 32 bits (4 bytes). IOWs,
since each lcluster can get 16 bits for its type and clusterofs, the
maximum supported lclustersize for compact 4B format is 16k (14 bits).

Fix this to enable compact 4B format for 16k lclusters (blocks), which
is tested on an arm64 server with 16k page size.

Fixes: 152a333a ("staging: erofs: add compacted compression indexes support")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230601112341.56960-1-hsiangkao@linux.alibaba.com

fs/erofs/zmap.c

@@ -129,7 +129,7 @@ static int unpack_compacted_index(struct z_erofs_maprecorder *m,
 	u8 *in, type;
 	bool big_pcluster;
 
-	if (1 << amortizedshift == 4)
+	if (1 << amortizedshift == 4 && lclusterbits <= 14)
 		vcnt = 2;
 	else if (1 << amortizedshift == 2 && lclusterbits == 12)
 		vcnt = 16;
@@ -231,7 +231,6 @@ static int compacted_load_cluster_from_disk(struct z_erofs_maprecorder *m,
 {
 	struct inode *const inode = m->inode;
 	struct erofs_inode *const vi = EROFS_I(inode);
-	const unsigned int lclusterbits = vi->z_logical_clusterbits;
 	const erofs_off_t ebase = sizeof(struct z_erofs_map_header) +
 		ALIGN(erofs_iloc(inode) + vi->inode_isize + vi->xattr_isize, 8);
 	unsigned int totalidx = erofs_iblks(inode);
@@ -239,9 +238,6 @@ static int compacted_load_cluster_from_disk(struct z_erofs_maprecorder *m,
 	unsigned int amortizedshift;
 	erofs_off_t pos;
 
-	if (lclusterbits != 12)
-		return -EOPNOTSUPP;
-
 	if (lcn >= totalidx)
 		return -EINVAL;