erspan: return PACKET_REJECT when the appropriate tunnel is not found

If erspan tunnel hasn't been established, we'd better send icmp port
unreachable message after receive erspan packets.

Fixes: 84e54fe0 ("gre: introduce native tunnel support for ERSPAN")
Cc: William Tu <u9012063@gmail.com>
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>