MFC b58cf1cb35ab:

Fix race condition in linuxkpi workqueue

Consider the following scenario:

1. A delayed_work struct in the WORK_ST_TIMER state.
2. Thread A calls mod_delayed_work()
3. Thread B (a callout thread) simultaneously calls

linux_delayed_work_timer_fn()

The following sequence of events is possible:

A: Call linux_cancel_delayed_work()
A: Change state from TIMER TO CANCEL
B: Change state from CANCEL to TASK
B: taskqueue_enqueue() the task
A: taskqueue_cancel() the task
A: Call linux_queue_delayed_work_on(). This is a no-op because the
state is WORK_ST_TASK.

As a result, the delayed_work struct will never be invoked. This is
causing address resolution in ib_addr.c to stop permanently, as it
never tries to reschedule a task that it thinks is already scheduled.

Fix this by introducing locking into the cancel path (which
corresponds with the lock held while the callout runs). This will
prevent the callout from changing the state of the task until the
cancel is complete, preventing the race.

Differential Revision: https://reviews.freebsd.org/D28420
Reviewed by: hselasky
MFC after: 2 months

(cherry picked from commit b58cf1cb35abc095d7fb9773630794b38bbc6b1d)