net: pktgen: Switch to use hrtimer_setup_sleeper_on_stack()

hrtimer_setup_sleeper_on_stack() replaces hrtimer_init_sleeper_on_stack()
to keep the naming convention consistent.

Convert the usage site over to it. The conversion was done with Coccinelle.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/c4b40b8fef250b6a325e1b8bd6057005fb3cb660.1730386209.git.namcao@linutronix.de