replace the magic numbers by offsetof(...) and sizeof(...), and add two
extra checks on xfs_check_ondisk_structs()

[dchinner: renamed header structures to be more descriptive]
Signed-off-by: NHou Tao <houtao1@huawei.com>
Reviewed-by: NCarlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>