block: null_blk: make fault-injection dynamically configurable per device

The null_blk driver has multiple driver-specific fault injection
mechanisms. Each fault injection configuration can only be specified by a
module parameter and cannot be reconfigured without reloading the driver.
Also, each configuration is common to all devices and is initialized every
time a new device is added.

This change adds the following subdirectories for each null_blk device.

/sys/kernel/config/nullb/<disk>/timeout_inject
/sys/kernel/config/nullb/<disk>/requeue_inject
/sys/kernel/config/nullb/<disk>/init_hctx_fault_inject

Each fault injection attribute can be dynamically set per device by a
corresponding file in these directories.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Link: https://lore.kernel.org/r/20230327143733.14599-3-akinobu.mita@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

null_blk: Move driver into its own directory

Move null_blk driver code into the new sub-directory
drivers/block/null_blk.

Suggested-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>