linuxkpi: Update struct irq_work

The previous llnode field is moved inside another field node.

This node field is a struct __call_single_node in Linux. Here, we
simply add an anonymous struct with the llnode field inside. That
field's new name is llist now.

V2: Use an anonymous union to keep the structure backward compatible

with drivers using the previous `llnode` field. This was suggested
by wufl@ and hselasky@. Thank you!

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D36955