fm10k: prevent race condition of __FM10K_SERVICE_SCHED

fm10k: prevent race condition of __FM10K_SERVICE_SCHED

Although very unlikely, it is possible that cancel_work_sync() may stop
the service_task before it actually started. In this case, the
__FM10K_SERVICE_SCHED bit will never be cleared. This results in the
service task being unable to reschedule in the future. Add a helper
function which sets the service disable bit, waits for the service task
to stop and clears the schedule bit, thus avoiding the race condition.
We know the schedule bit is safe to clear because the cancel_work_sync()
guarantees the service task is not running.

Add a helper function also to restart the service task, for symmetry.
This is not strictly needed but helps the mental model of how to stop
and start the service task.

This race could only happen in fm10k_suspend/fm10k_resume as this is the
only place where the service task is actually restarted. Thus,
suspend/resume testing would be ideal. However, note that the chance of
this happening is very slim as the service event is scheduled for
immediate execution, and you would have to trigger a suspend at almost
the exact same time as the service task was scheduled.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>