esp6: fix memleak on error path in esp6_input

This ought to be an omission in e6194923 ("esp: Fix memleaks on error
paths."). The memleak on error path in esp6_input is similar to esp_input
of esp4.

Fixes: e6194923 ("esp: Fix memleaks on error paths.")
Fixes: 3f297707 ("ipsec: check return value of skb_to_sgvec always")
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>