lightnvm: pblk: remove some unnecessary NULL checks

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: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>