xfrm: policy: fix sparse warnings in xfrm_policy_{init,fini}

In xfrm_policy_init:
add rcu_assign_pointer to fix warning:
net/xfrm/xfrm_policy.c:4238:29: warning: incorrect type in assignment (different address spaces)
net/xfrm/xfrm_policy.c:4238:29:    expected struct hlist_head [noderef] __rcu *table
net/xfrm/xfrm_policy.c:4238:29:    got struct hlist_head *

add rcu_dereference_protected to silence warning:
net/xfrm/xfrm_policy.c:4265:36: warning: incorrect type in argument 1 (different address spaces)
net/xfrm/xfrm_policy.c:4265:36:    expected struct hlist_head *n
net/xfrm/xfrm_policy.c:4265:36:    got struct hlist_head [noderef] __rcu *table

The netns is being created, no concurrent access is possible yet.

In xfrm_policy_fini, net is going away, there shouldn't be any
concurrent changes to the hashtables, so we can use
rcu_dereference_protected to silence warnings:
net/xfrm/xfrm_policy.c:4291:17: warning: incorrect type in argument 1 (different address spaces)
net/xfrm/xfrm_policy.c:4291:17:    expected struct hlist_head const *h
net/xfrm/xfrm_policy.c:4291:17:    got struct hlist_head [noderef] __rcu *table
net/xfrm/xfrm_policy.c:4292:36: warning: incorrect type in argument 1 (different address spaces)
net/xfrm/xfrm_policy.c:4292:36:    expected struct hlist_head *n
net/xfrm/xfrm_policy.c:4292:36:    got struct hlist_head [noderef] __rcu *table

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