nvmet: get transport reference for passthru ctrl

Grab a reference to the transport driver to ensure it can't be unloaded
while a passthrough controller is active.

Fixes: c1fef73f ("nvmet: add passthru code to process commands")
Reported-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>