backing-file: convert to using fops->splice_write

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>

fs/backing-file.c

@@ -303,13 +303,16 @@ ssize_t backing_file_splice_write(struct pipe_inode_info *pipe,
 	if (WARN_ON_ONCE(!(out->f_mode & FMODE_BACKING)))
 		return -EIO;
 
+	if (!out->f_op->splice_write)
+		return -EINVAL;
+
 	ret = file_remove_privs(ctx->user_file);
 	if (ret)
 		return ret;
 
 	old_cred = override_creds(ctx->cred);
 	file_start_write(out);
-	ret = iter_file_splice_write(pipe, out, ppos, len, flags);
+	ret = out->f_op->splice_write(pipe, out, ppos, len, flags);
 	file_end_write(out);
 	revert_creds(old_cred);