xfs: removed unused error variable from xchk_refcountbt_rec

Removed unused error variable. Instead of using error variable,
returned the value directly as it wasn't updated.

Signed-off-by: Aliasgar Surti <aliasgar.surti500@gmail.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>