xfs_scrub: create a new category for unfixable errors

There's nothing that xfs_scrub (or XFS) can do about media errors for
data file blocks -- the data are gone.  Create a new category for these
unfixable errors so that we don't advise the user to take further action
that won't fix the problem.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>