igb: remove rx_ps_hdr_len

This patch removes the rx_ps_hdr_len which isn't really needed since we can
now use rx_buffer_len less than 1K to indicate that we are in a packet split
mode.  We also don't need it since we always use a half page for the data
buffers when receiving so we always know the size to map/unmap.

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>