net: xfrm: fix memory leak in xfrm_user_policy()

if xfrm_get_translator() failed, xfrm_user_policy() return without
freeing 'data', which is allocated in memdup_sockptr().

Fixes: 96392ee5 ("xfrm/compat: Translate 32-bit user_policy from sockptr")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>