nvme-fc: do not wait in vain when unloading module

In the Linux kernel, the following vulnerability has been resolved:

nvme-fc: do not wait in vain when unloading module

The module exit path has race between deleting all controllers and

freeing 'left over IDs'. To prevent double free a synchronization

between nvme_delete_ctrl and ida_destroy has been added by the initial

commit.

There is some logic around trying to prevent from hanging forever in

wait_for_completion, though it does not handling all cases. E.g.

blktests is able to reproduce the situation where the module unload

hangs forever.

If we completely rely on the cleanup code executed from the

nvme_delete_ctrl path, all IDs will be freed eventually. This makes

calling ida_destroy unnecessary. We only have to ensure that all

nvme_delete_ctrl code has been executed before we leave

nvme_fc_exit_module. This is done by flushing the nvme_delete_wq

workqueue.

While at it, remove the unused nvme_fc_wq workqueue too.