net:ipv6: check return value of pskb_trim()

goto tx_err if an unexpected result is returned by pskb_tirm()
in ip6erspan_tunnel_xmit().

Fixes: 5a963eb6

 ("ip6_gre: Add ERSPAN native tunnel support")
Signed-off-by: Yuanjun Gong <ruc_gongyuanjun@163.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>