fs: assert that open_mutex isn't held over holder ops

fs: assert that open_mutex isn't held over holder ops

With recent block level changes we should never be in a situation where
we hold disk->open_mutex when calling into these helpers. So assert that
in the code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231017184823.1383356-6-hch@lst.de
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>