vrf: Fix use-after-free in vrf_xmit

vrf: Fix use-after-free in vrf_xmit

KASAN detected a use-after-free:

[  269.467067] BUG: KASAN: use-after-free in vrf_xmit+0x7f1/0x827 [vrf] at addr ffff8800350a21c0
[  269.467067] Read of size 4 by task ssh/1879
[  269.467067] CPU: 1 PID: 1879 Comm: ssh Not tainted 4.10.0+ #249
[  269.467067] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
[  269.467067] Call Trace:
[  269.467067]  dump_stack+0x81/0xb6
[  269.467067]  kasan_object_err+0x21/0x78
[  269.467067]  kasan_report+0x2f7/0x450
[  269.467067]  ? vrf_xmit+0x7f1/0x827 [vrf]
[  269.467067]  ? ip_output+0xa4/0xdb
[  269.467067]  __asan_load4+0x6b/0x6d
[  269.467067]  vrf_xmit+0x7f1/0x827 [vrf]
...

Which corresponds to the skb access after xmit handling. Fix by saving
skb->len and using the saved value to update stats.

Fixes: 193125dbd8eb2 ("net: Introduce VRF device driver")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>