net: ipv6: fix return value check in esp_remove_trailer

In esp_remove_trailer(), to avoid an unexpected result returned by
pskb_trim, we should check the return value of pskb_trim().

Signed-off-by: Ma Ke <make_ruc2021@163.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>