ext4: Use helper function to access group descriptor and its fields

The descriptor size is variable, thus array indices are not generically
applicable. The larger group descriptors also contain e.g. high parts
of block numbers, which have to be read and written.

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

include/ext4fs.h

@@ -98,7 +98,6 @@ struct ext_filesystem {
 	/* Superblock */
 	struct ext2_sblock *sb;
 	/* Block group descritpor table */
-	struct ext2_block_group *bgd;
 	char *gdtable;
 
 	/* Block Bitmap Related */