Smatch complains that flush_workqueue() dereferences the work queue
pointer but then we check if it's NULL on the next line when it's too
late.  These NULL checks can be removed because the module won't load if
we can't allocate the work queues.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NMatias Bjørling <mb@lightnvm.io>
Signed-off-by: NJens Axboe <axboe@kernel.dk>