iomap: use a new variable to handle the written bytes in iomap_write_iter()

In iomap_write_iter(), the status variable used to receive the return
value from iomap_write_end() is confusing, replace it with a new written
variable to represent the written bytes in each cycle, no logic changes.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://lore.kernel.org/r/20240320110548.2200662-8-yi.zhang@huaweicloud.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>