KVM: x86: Use kvfree_rcu() to free old optimized APIC map

Use kvfree_rcu() to free the old optimized APIC instead of open coding a
rough equivalent via call_rcu() and a callback function.

Note, there is a subtle function change as rcu_barrier() doesn't wait on
kvfree_rcu(), but does wait on call_rcu().  Not forcing rcu_barrier() to
wait is safe and desirable in this case, as KVM doesn't care when an old
map is actually freed.  In fact, using kvfree_rcu() fixes a largely
theoretical use-after-free.  Because KVM _doesn't_ do rcu_barrier() to
wait for kvm_apic_map_free() to complete, if KVM-the-module is unloaded in
the RCU grace period before kvm_apic_map_free() is invoked, KVM's callback
could run after module unload.

Signed-off-by: Li RongQing <[email protected]>
Reviewed-by: Neeraj Upadhyay <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[sean: rework changelog, call out rcu_barrier() interaction]
Signed-off-by: Sean Christopherson <[email protected]>

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