net: Avoid the IPI to free the

skb_attempt_defer_free() collects a skbs, which was allocated on a
remote CPU, on a per-CPU list. These skbs are either freed on that
remote CPU once the CPU enters NET_RX or an remote IPI function is
invoked in to raise the NET_RX softirq if a threshold of pending skb has
been exceeded.
This remote IPI can cause the wakeup of ksoftirqd on PREEMPT_RT if the
remote CPU idle was idle. This is undesired because once the ksoftirqd
is running it will acquire all pending softirqs and they will not be
executed as part of the threaded interrupt until ksoftird goes idle
again.

To void all this, schedule the deferred clean up from a worker.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>