ocfs2: fix leaf start calculation in ocfs2_dx_dir_rebalance()

ocfs2_dx_dir_rebalance() is passed the block offset of a dx leaf which needs
rebalancing. Since we rebalance an entire cluster at a time however, this
function needs to calculate the beginning of that cluster, in blocks. The
calculation was wrong, which would result in a read of non-leaf blocks. Fix
the calculation by adding ocfs2_block_to_cluster_start() which is a more
straight-forward way of determining this.
Reported-by: NTristan Ye <tristan.ye@oracle.com>
Signed-off-by: NMark Fasheh <mfasheh@suse.com>

fs/ocfs2/dir.c

@@ -3941,8 +3941,7 @@ static int ocfs2_dx_dir_rebalance(struct ocfs2_super *osb, struct inode *dir,
 		goto out_commit;
 	}
 
-	orig_leaves_start = leaf_blkno & ~(osb->s_clustersize_bits -
-				      osb->sb->s_blocksize_bits);
+	orig_leaves_start = ocfs2_block_to_cluster_start(dir->i_sb, leaf_blkno);
 	ret = ocfs2_read_dx_leaves(dir, orig_leaves_start, num_dx_leaves,
 				   orig_dx_leaves);
 	if (ret) {

fs/ocfs2/ocfs2.h

@@ -582,6 +582,16 @@ static inline u64 ocfs2_clusters_to_bytes(struct super_block *sb,
 	return (u64)clusters << OCFS2_SB(sb)->s_clustersize_bits;
 }
 
+static inline u64 ocfs2_block_to_cluster_start(struct super_block *sb,
+					       u64 blocks)
+{
+	int bits = OCFS2_SB(sb)->s_clustersize_bits - sb->s_blocksize_bits;
+	unsigned int clusters;
+
+	clusters = ocfs2_blocks_to_clusters(sb, blocks);
+	return (u64)clusters << bits;
+}
+
 static inline u64 ocfs2_align_bytes_to_clusters(struct super_block *sb,
 						u64 bytes)
 {