xen: use time_is_before_eq_jiffies() instead of open coding it

Use the helper function time_is_{before,after}_jiffies() to improve
code readability.

Signed-off-by: Wang Qing <wangqing@vivo.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/1646018104-61415-1-git-send-email-wangqing@vivo.com


Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>