LinuxKPI: Implement irq_work_sync() routine.

irq_work_sync() performs draining of irq_work task.
Required by drm-kmod.

Reviewed by: hselasky
Differential revision: https://reviews.freebsd.org/D30818

(cherry picked from commit 864b11007ab3de9f65f28b730507225396c38b9e)