[ROSE]: Fix dereference of skb pointer after free.

If rose_route_frame return success we'll dereference a stale pointer.
Likely this is only going to result in bad statistics for the ROSE
interface.

This fixes coverity 946.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>