super: move lockdep assert

Fix braino and move the lockdep assertion after put_super() otherwise we
risk a use-after-free.

Fixes: 2c18a63b ("super: wait until we passed kill super")
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Message-Id: <20230828-vfs-super-fixes-v1-1-b37a4a04a88f@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>