Plugs never insert at head, so don't plug for head insertions.

Fixes: 1c2d2fff ("block: wire-up support for passthrough plugging")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20230413064057.707578-2-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>