vrf: fix NULL dereference in vrf_finish_output()

The "skb" pointer is NULL on this error path so we can't dereference it.
Use "dev" instead.

Fixes: 14ee70ca ("vrf: use skb_expand_head in vrf_finish_output")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20210806150435.GB15586@kili


Signed-off-by: Jakub Kicinski <kuba@kernel.org>