xfrm: Replace hrtimer tasklet with softirq hrtimer

Switch the timer to HRTIMER_MODE_SOFT, which executed the timer
callback in softirq context and remove the hrtimer_tasklet.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Link: https://lkml.kernel.org/r/20190301224821.29843-3-bigeasy@linutronix.de