netdevsim: fix UaF when counting Tx stats

skb may be freed as soon as we put it on the rx queue.
Use the len variable like the code did prior to the conversion.

Fixes: f9e2511d ("netdevsim: migrate to dstats stats collection")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>