igbvf: fix memory leak when ring size changed while interface down

This patch resolves a memory leak which occurs while changing the ring size
while the interface is down.

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>