gfs2: hole_size improvement

Reimplement function hole_size based on a generic function for walking
the metadata tree and rename hole_size to gfs2_hole_size.  While
previously, multiple invocations of hole_size were sometimes needed to
walk across the entire hole, the new implementation always returns the
entire hole at once (provided that the caller is interested in the total
size).

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>