ravb: implement MTU change while device is up

Pre-allocates buffers sufficient for the maximum supported MTU (2026) in
order to eliminate the possibility of resource exhaustion when changing the
MTU while the device is up.

Signed-off-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>