extent scan: pick the right block groups for mixed-bg filesystems

extent scan: pick the right block groups for mixed-bg filesystems

The progress indicator was failing on a mixed-bg filesystem because those
filesystems have block groups which have both _DATA and _METADATA bits,
and the filesystem size calculation was excluding block groups that have
_METADATA set.  It should exclude block groups that have _DATA not set.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>