xfrm: Wait for RCU readers during policy netns exit

xfrm_policy_fini() frees the policy_bydst hash tables after flushing the
policy work items and deleting all policies, but it does not wait for
concurrent RCU readers to leave their read-side critical sections first.

The policy_bydst tables are published via rcu_assign_pointer() and are
looked up through rcu_dereference_check(), so netns teardown must also
wait for an RCU grace period before freeing the table memory.

Fix this by adding synchronize_rcu() before freeing the policy hash tables.

Fixes: e1e551bc5630 ("xfrm: policy: prepare policy_bydst hash for rcu lookups")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Reviewed-by: Florian Westphal <fw@strlen.de>