Convert to use block group accessor functions

Convert to use block group accessor functions

Convert direct accesses to use the following block group accessor
functions: ext2fs_block_bitmap_loc(), ext2fs_inode_bitmap_loc(),
ext2fs_inode_table_loc(), ext2fs_bg_itable_unused(),
ext2fs_block_bitmap_loc_set(), ext2fs_inode_bitmap_loc_set(),
ext2fs_inode_table_loc_set(), ext2fs_bg_free_inodes_count(),
ext2fs_ext2fs_bg_used_dirs_count(), ext2fs_bg_free_inodes_count_set(),
ext2fs_bg_free_blocks_count_set(), ext2fs_bg_used_dirs_count_set()

Signed-off-by: Valerie Aurora Henson <vaurora@redhat.com>
Signed-off-by: Nick Dokos <nicholas.dokos@hp.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>