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>

net/xfrm/xfrm_user.c

@@ -3058,7 +3058,7 @@ static inline unsigned int xfrm_sa_len(struct xfrm_state *x)
 	if (x->props.extra_flags)
 		l += nla_total_size(sizeof(x->props.extra_flags));
 	if (x->xso.dev)
-		 l += nla_total_size(sizeof(x->xso));
+		 l += nla_total_size(sizeof(struct xfrm_user_offload));
 	if (x->props.smark.v | x->props.smark.m) {
 		l += nla_total_size(sizeof(x->props.smark.v));
 		l += nla_total_size(sizeof(x->props.smark.m));