ext4: Use correct descriptor size when reading the block group descriptor

The correct descriptor size must be used when calculating offsets, and
also to read the correct amount of data.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>

fs/ext4/ext4_common.c

@@ -1527,20 +1527,20 @@ static int ext4fs_blockgroup
 	long int blkno;
 	unsigned int blkoff, desc_per_blk;
 	int log2blksz = get_fs()->dev_desc->log2blksz;
+	int desc_size = get_fs()->gdsize;
 
-	desc_per_blk = EXT2_BLOCK_SIZE(data) / sizeof(struct ext2_block_group);
+	desc_per_blk = EXT2_BLOCK_SIZE(data) / desc_size;
 
 	blkno = le32_to_cpu(data->sblock.first_data_block) + 1 +
 			group / desc_per_blk;
-	blkoff = (group % desc_per_blk) * sizeof(struct ext2_block_group);
+	blkoff = (group % desc_per_blk) * desc_size;
 
 	debug("ext4fs read %d group descriptor (blkno %ld blkoff %u)\n",
 	      group, blkno, blkoff);
 
 	return ext4fs_devread((lbaint_t)blkno <<
 			      (LOG2_BLOCK_SIZE(data) - log2blksz),
-			      blkoff, sizeof(struct ext2_block_group),
-			      (char *)blkgrp);
+			      blkoff, desc_size, (char *)blkgrp);
 }
 
 int ext4fs_read_inode(struct ext2_data *data, int ino, struct ext2_inode *inode)