xfrm: state: fix sparse warnings around XFRM_STATE_INSERT

We're under xfrm_state_lock in all those cases, use
xfrm_state_deref_prot(state_by*) to avoid sparse warnings:

net/xfrm/xfrm_state.c:2597:25: warning: cast removes address space '__rcu' of expression
net/xfrm/xfrm_state.c:2597:25: warning: incorrect type in argument 2 (different address spaces)
net/xfrm/xfrm_state.c:2597:25:    expected struct hlist_head *h
net/xfrm/xfrm_state.c:2597:25:    got struct hlist_head [noderef] __rcu *

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