Incorrect maximum DVA value in DDE_GET_NDVAS()

The conditional was reversed which caused garbage values to be used when
calculating dds_ref_dsize.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes #7234