ixgbe: Lock RSS seed, move rx_buf_len to the rx_ring

This locks the seed down so loading/unloading the driver will present
predictable hashing from RSS.  Also move the rx_buf_len out of the adapter
struct, and into the Rx ring struct.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>