net: erspan: remove md NULL check

The 'md' is allocated from 'tun_dst = ip_tun_rx_dst' and
since we've checked 'tun_dst', 'md' will never be NULL.
The patch removes it at both ipv4 and ipv6 erspan.

Fixes: afb4c97d ("ip6_gre: fix potential memory leak in ip6erspan_rcv")
Fixes: 50670b6e

 ("ip_gre: fix potential memory leak in erspan_rcv")
Cc: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>