super: simplify user_get_super()

Make it easier to read and remove one level of identation.

Link: https://lore.kernel.org/r/20250329-work-freeze-v2-2-a47af37ecc3d@kernel.org


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>