null_blk: add module parameters for 4 options

null_blk: add module parameters for 4 options

Add as module parameters these options:

memory_backed
discard
mbps
cache_size

Previously these could only be set via configfs.

Still missing is bad_blocks.

The kernel test robot found a documentation formatting issue in v1 of
this patch.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>
Link: https://lore.kernel.org/r/20220708174943.87787-2-vincent.fu@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>