xfs: Use defines for CRC offsets in all cases

Some calls to crc functions used useful #defines,
others used awkward offsetof() constructs.

Switch them all to #define to make things a bit cleaner.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>