dirgen: use atomic post-increment operator rather than fetch_add()
dirgen: use atomic post-increment operator rather than fetch_add()

Just because it looks nicer...

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3