i40e: Fix preempt count leak in napi poll tracepoint

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

i40e: Fix preempt count leak in napi poll tracepoint

Using get_cpu() in the tracepoint assignment causes an obvious preempt

count leak because nothing invokes put_cpu() to undo it:

softirq: huh, entered softirq 3 NET_RX with preempt_count 00000100, exited with 00000101?

This clearly has seen a lot of testing in the last 3+ years...

Use smp_processor_id() instead.