ice: introduce legacy Rx flag

Add an ethtool "legacy-rx" priv flag for toggling the Rx path. This
control knob will be mainly used for build_skb usage as well as buffer
size/MTU manipulation.

In preparation for adding build_skb support in a way that it takes
care of how we set the values of max_frame and rx_buf_len fields of
struct ice_vsi. Specifically, in this patch mentioned fields are set to
values that will allow us to provide headroom and tailroom in-place.

This can be mostly broken down onto following:
- for legacy-rx "on" ethtool control knob, old behaviour is kept;
- for standard 1500 MTU size configure the buffer of size 1536, as
  network stack is expecting the NET_SKB_PAD to be provided and
  NET_IP_ALIGN can have a non-zero value (these can be typically equal
  to 32 and 2, respectively);
- for larger MTUs go with max_frame set to 9k and configure the 3k
  buffer in case when PAGE_SIZE of underlying arch is less than 8k; 3k
  buffer is implying the need for order 1 page, so that our page
  recycling scheme can still be applied;

With that said, substitute the hardcoded ICE_RXBUF_2048 and PAGE_SIZE
values in DMA API that we're making use of with rx_ring->rx_buf_len and
ice_rx_pg_size(rx_ring). The latter is an introduced helper for
determining the page size based on its order (which was figured out via
ice_rx_pg_order). Last but not least, take care of truesize calculation.

In the followup patch the headroom/tailroom computation logic will be
introduced.

This change aligns the buffer and frame configuration with other Intel
drivers, most importantly with iavf.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>