All callers can safely block. Kill the atomic/block argument, and
remove the argument from all callers.

Reviewed-by: Bart Van Assche <Bart.VanAssche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>