ring: Simplify calculation of number of frames in a tpacket ring

ring: Simplify calculation of number of frames in a tpacket ring

The number of frames in a tpacket ring (ring->layout.tp_frame_nr) is
currently calculated as:

  tp_frame_nr = tp_block_size / tp_frame_size * tp_block_nr

Substituting tp_block_nr with 'size / tp_block_size' (as calculated in
the line above), we get:

  tp_frame_nr = tp_block_size / tp_frame_size * (size / tp_block_size)

and realize that we can omit tp_block_size as it cancels out, leading
to:

  tp_frame_nr = 1 / tp_frame_size * (size / 1)
              = size / tp_frame_size

Adjust the calculation in setup_ring_layout_generic() accordingly.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>