mptsas_cleanup_fw_event_q() uses in_interrupt() to determine if it is safe
to cancel a worker item.

Aside of that in_interrupt() is deprecated as it does not provide what the
name suggests. It covers more than hard/soft interrupt servicing context
and is semantically ill defined.

Looking closer there are a few problems with the current construct:

 - It could be invoked from an interrupt handler / non-blocking context
   because cancel_delayed_work() has no such restriction. Also,
   mptsas_free_fw_event() has no such restriction.

 - The list is accessed unlocked. It may dequeue a valid work-item but at
   the time of invoking cancel_delayed_work() the memory may be released or
   reused because the worker has already run.

mptsas_cleanup_fw_event_q() is invoked via mptsas_shutdown() which is
always invoked from preemtible context on device shutdown.  It is also
invoked via mptsas_ioc_reset(, MPT_IOC_POST_RESET) which is a
MptResetHandlers callback. The only caller here are mpt_SoftResetHandler(),
mpt_HardResetHandler() and mpt_Soft_Hard_ResetHandler(). All these
functions have a `sleepFlag' argument and each caller uses caller uses
`CAN_SLEEP' here and according to current documentation: | @sleepFlag:
Indicates if sleep or schedule must be called

So it is safe to sleep.

Add mptsas_hotplug_event::users member. Initialize it to one by default so
mptsas_free_fw_event() will free the memory.  mptsas_cleanup_fw_event_q()
will increment its value for items it dequeues and then it may keep a
pointer after dropping the lock.  Invoke cancel_delayed_work_sync() to
cancel the work item and wait if the worker is currently busy. Free the
memory afterwards since it owns the last reference to it.

Link: https://lore.kernel.org/r/20201126132952.2287996-15-bigeasy@linutronix.de


Cc: Sathya Prakash <sathya.prakash@broadcom.com>
Cc: Sreekanth Reddy <sreekanth.reddy@broadcom.com>
Cc: Suganath Prabu Subramani <suganath-prabu.subramani@broadcom.com>
Cc: MPT-FusionLinux.pdl@broadcom.com
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>