xfrm: state: add xfrm_state_deref_prot to state_by* walk under lock

We're under xfrm_state_lock for all those walks, we can use
xfrm_state_deref_prot to silence sparse warnings such as:

net/xfrm/xfrm_state.c:933:17: warning: dereference of noderef expression

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