block: factor out common part in blkdev_fallocate()

Only the flags passed to blkdev_issue_zeroout() differ among the two
zeroing branches in blkdev_fallocate(). Therefore, do cleanup by
factoring them out.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://lore.kernel.org/20250619111806.3546162-8-yi.zhang@huaweicloud.com


Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>