xfrm: fix a small bug in xfrm_sa_len()

stable inclusion
from linux-4.19.226
commit fcb8f503d607d18b9fc69613e38358aae9f9fa99

--------------------------------

[ Upstream commit 7770a39d ]

copy_user_offload() will actually push a struct struct xfrm_user_offload,
which is different than (struct xfrm_state *)->xso
(struct xfrm_state_offload)

Fixes: d77e38e6 ("xfrm: Add an IPsec hardware offloading API")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>