xfrm: state: fix sparse warnings on xfrm_state_hold_rcu

In all callers, x is not an __rcu pointer. We can drop the annotation to
avoid sparse warnings:

net/xfrm/xfrm_state.c:58:39: warning: incorrect type in argument 1 (different address spaces)
net/xfrm/xfrm_state.c:58:39:    expected struct refcount_struct [usertype] *r
net/xfrm/xfrm_state.c:58:39:    got struct refcount_struct [noderef] __rcu *
net/xfrm/xfrm_state.c:1166:42: warning: incorrect type in argument 1 (different address spaces)
net/xfrm/xfrm_state.c:1166:42:    expected struct xfrm_state [noderef] __rcu *x
net/xfrm/xfrm_state.c:1166:42:    got struct xfrm_state *[assigned] x
(repeated for each caller)

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>