xfs: sanity check ag header values in xrep_calc_ag_resblks

Check the values we read in from the AG headers when calculating the
block reservations for a repair transaction.  If they're obviously
wrong, substitute worst case assumptions (rather than ENOSPC on a bogus
reservation request).

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>