If md is NULL, tun_dst must be freed, otherwise it will cause memory
leak.

Fixes: ef7baf5e ("ip6_gre: add ip6 erspan collect_md mode")
Cc: William Tu <u9012063@gmail.com>
Signed-off-by: NHaishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>