libext2fs: use ext2fs_group_first_block() instead of open-coded equivalent

Use ext2fs_group_first_block() instead of the open-coded equivalent in
ext2fs_super_and_bgd_loc() and ext2fs_descriptor_block_loc().

Signed-off-by: Valerie Clement <valerie.clement@bull.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
2 files changed