e1000: remove races when changing mtu

this patch fixes a bug that occurs when routing packets and simultaneously
changing the mtu.  the rx_buffer_len variable is used during the rx cleanup
and if that changes on the fly without stopping traffic bad things happen

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>