backing_file_user_path(): constify struct path *

Callers never use the resulting pointer to modify the struct path it
points to (nor should they).

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>