erspan: fix error handling for erspan tunnel

When processing icmp unreachable message for erspan tunnel, tunnel id
should be erspan_net_id instead of ipgre_net_id.

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>