ip_gre: remove an extra dst_release()

commit 68c33163 (v4 GRE: Add TCP segmentation offload for GRE)
introduced a bug in error path.

dst is attached to skb, so will be released when skb is freed.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>