super: skip dying superblocks early

Make all iterators uniform by performing an early check whether the
superblock is dying.

Link: https://lore.kernel.org/r/20250329-work-freeze-v2-3-a47af37ecc3d@kernel.org


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