iomap: make iomap_write_end() return a boolean

For now, we can make sure iomap_write_end() always return 0 or copied
bytes, so instead of return written bytes, convert to return a boolean
to indicate the copied bytes have been written to the pagecache.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://lore.kernel.org/r/20240320110548.2200662-9-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>