scsi: zfcp: Lift Input Queue tasklet from qdio

scsi: zfcp: Lift Input Queue tasklet from qdio

Shift the IRQ tasklet processing from the qdio layer into zfcp.  This will
allow for a good amount of cleanups in qdio, and provides future
opportunity to improve the IRQ processing inside zfcp.

We continue to use the qdio layer's internal tasklet/timer mechanism
(ie. scan_threshold etc) to check for Request Queue completions.  Initially
we planned to check for such completions after inspecting the Response
Queue - this should typically work, but there's a theoretical race where
the device only presents the Request Queue completions _after_ all Response
Queue processing has finished.  If the Request Queue is then also
_completely_ full, we could send no further IOs and thus get no interrupt
that would trigger an inspection of the Request Queue.  So for now stick to
the old model, where we can trust that such a race would be recovered by
qdio's internal timer.

Code-flow wise, this establishes two levels of control:

1. The qdio layer will only deliver IRQs to the device driver if the
   QDIO_IRQ_DISABLED flag is cleared. zfcp manages this through
   qdio_start_irq() / qdio_stop_irq(). The initial state is DISABLED, and
   zfcp_qdio_open() schedules zfcp's IRQ tasklet once during startup to
   explicitly enable IRQ delivery.

2. The zfcp tasklet is initialized with tasklet_disable(), and only gets
   enabled once we open the qdio device.  When closing the qdio device, we
   must disable the tasklet _before_ disabling IRQ delivery (otherwise a
   concurrently running tasklet could re-enable IRQ delivery after we
   disabled it).

   A final tasklet_kill() during teardown ensures that no lingering
   tasklet_schedule() is still accessing the tasklet structure.

Link: https://lore.kernel.org/r/94a765211c48b74a7b91c5e60b158de01db98d43.1603908167.git.bblock@linux.ibm.com
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>