xfrm: fix a small bug in xfrm_sa_len()

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>