virtio: mem: Switch to use hrtimer_setup()

hrtimer_setup() takes the callback function pointer as argument and
initializes the timer completely.

Replace hrtimer_init() and the open coded initialization of
hrtimer::function with the new setup mechanism.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://lore.kernel.org/all/4a911503d16684592b59a16d8ade97e42df64a54.1738746904.git.namcao@linutronix.de