igb: move rx_buffer_len into the ring structure

This patch moves the rx_buffer_len value into the ring structure.  This allows
greater flexibility and the option of doing things such as supporting packet
split only on some queues, or enabling virtualization.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>