xfs: check the uniqueness of the AGFL entries

Make sure we don't list a block twice in the agfl by copying the
contents of the AGFL to an array, sorting it, and looking for
duplicates.  We can easily check that the number of agfl entries we see
actually matches the flcount, so do that too.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>