x86/kvm: Alloc dummy async #PF token outside of raw spinlock

Drop the raw spinlock in kvm_async_pf_task_wake() before allocating the
the dummy async #PF token, the allocator is preemptible on PREEMPT_RT
kernels and must not be called from truly atomic contexts.

Opportunistically document why it's ok to loop on allocation failure,
i.e. why the function won't get stuck in an infinite loop.

Reported-by: Yajun Deng <yajun.deng@linux.dev>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>