allow to run most tests with built-in null_blk

Christoph Hellwig
2022-06-03T04:56:05Z

Hi Shin'ichiro,

this series updates most tests that use null_blk so that they can
work with a built-in null_blk driver. The onces that require
shared_tags or failure injection, which currently can only be
controlled through module parameters still require a module until
the kernel is updated (which I plan to look into).