xfs: refactor refcount record usage in xchk_refcountbt_rec

Consolidate the open-coded xfs_refcount_irec fields into an actual
struct and use the existing _btrec_to_irec to decode the ondisk record.
This will reduce code churn in the next patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>