backing-file: convert to using fops->splice_write

[ Upstream commit 996b37da1e0f51314d4186b326742c2a95a9f0dd ]

Filesystems may define their own splice write. Therefore, use the file
fops instead of invoking iter_file_splice_write() directly.

Signed-off-by: Ed Tsai <ed.tsai@mediatek.com>
Link: https://lore.kernel.org/r/20240708072208.25244-1-ed.tsai@mediatek.com
Fixes: 5ca73468

 ("fuse: implement splice read/write passthrough")
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>