linuxkpi: Update `struct irq_work`

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