ipv4: ip_gre: fix return value check in erspan_xmit()

goto free_skb if an unexpected result is returned by pskb_tirm()
in erspan_xmit().

Signed-off-by: Yuanjun Gong <ruc_gongyuanjun@163.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>