xfrm: state: initialize state_ptrs earlier in xfrm_state_find

In the Linux kernel, the following vulnerability has been resolved:

xfrm: state: initialize state_ptrs earlier in xfrm_state_find

In case of preemption, xfrm_state_look_at will find a different pcpu_id and look up states for that other CPU. If we matched a state for CPU2 in the state_cache while the lookup started on CPU1, we will jump to "found", but the "best" state that we got will be ignored and we will enter the "acquire" block. This block uses state_ptrs, which isn't initialized at this point.

Let's initialize state_ptrs just after taking rcu_read_lock. This will also prevent a possible misuse in the future, if someone adjusts this function.