xfrm: state: silence sparse warnings during netns exit

Silence sparse warnings in xfrm_state_fini:
net/xfrm/xfrm_state.c:3327:9: warning: incorrect type in argument 1 (different address spaces)
net/xfrm/xfrm_state.c:3327:9:    expected struct hlist_head const *h
net/xfrm/xfrm_state.c:3327:9:    got struct hlist_head [noderef] __rcu *state_byseq

Add xfrm_state_deref_netexit() to wrap those calls. The netns is going
away, we don't have to worry about the state_by* pointers being
changed behind our backs.

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