nvmet: update AEN list and array at one place

All async events are enqueued via nvmet_add_async_event() which
updates the ctrl->async_event_cmds[] array and additionally an struct
nvmet_async_event is added to the ctrl->async_events list.

Under normal operations the nvmet_async_event_work() updates again
the ctrl->async_event_cmds and removes the corresponding struct
nvmet_async_event from the list again. Though nvmet_sq_destroy() could
be called which calls nvmet_async_events_free() which only updates the
ctrl->async_event_cmds[] array.

Add new functions nvmet_async_events_process() and
nvmet_async_events_free() to process async events, update an array and
the list.

When we destroy submission queue after clearing the aen present on
the ctrl->async list we also loop over ctrl->async_event_cmds[] for
any requests posted by the host for which we don't have the AEN in
the ctrl->async_events list by calling nvmet_async_event_process()
and nvmet_async_events_free().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
[chaitanya.kulkarni@wdc.com
 * Loop over and clear out outstanding requests
 * Update changelog
]
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>