firewire: net: fix use after free in fwnet_finish_incoming_packet()

The netif_rx() function frees the skb so we can't dereference it to
save the skb->len.

Signed-off-by: Zhang Shurong <zhang_shurong@foxmail.com>
Link: https://lore.kernel.org/r/tencent_3B3D24B66ED66A6BB73CC0E63C6A14E45109@qq.com


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>