LinuxKPI: Do not use forward declaration for struct llist_node

in linux/irq_work.h as irq_work structure contains full version of
llist_node rather than reference to it.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35052

(cherry picked from commit bec4576e4c194a3b3bc8fb8cce613d7088dbfb2c)