backing-file: fix backing_file_open() kerneldoc parameter

backing-file: fix backing_file_open() kerneldoc parameter

The kerneldoc for backing_file_open() documented a @user_path argument,
but the function takes const struct file *user_file. The user
path is derived as &user_file->f_path.

Update the @-tag to @user_file and adjust the description accordingly.
Also fix the "reuqested" typo to 'requested' in the old comment.

Signed-off-by: Li Wang <liwang@kylinos.cn>
Link: https://patch.msgid.link/20260528104208.395757-1-liwang@kylinos.cn
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>