block: use kstrtoul in flag_store

Use the text to integer helper that has error handling and doesn't modify
the input pointer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Link: https://lore.kernel.org/r/20240613084839.1044015-9-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>