ocfs2: Use ocfs2_rec_clusters in ocfs2_adjust_adjacent_records.

In ocfs2_adjust_adjacent_records, we will adjust adjacent records
according to the extent_list in the lower level. But actually
the lower level tree will either be a leaf or a branch. If we only
use ocfs2_is_empty_extent we will meet with some problem if the lower
tree is a branch (tree_depth > 1). So use !ocfs2_rec_clusters instead.
And actually only the leaf record can have holes. So add a BUG_ON
for non-leaf branch.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>