null_blk: add option for managing virtual boundary

This will enable changing the virtual boundary of null blk devices. For
now, null blk devices didn't have any restriction on the scatter/gather
elements received from the block layer. Add a module parameter and a
configfs option that will control the virtual boundary. This will
enable testing the efficiency of the block layer bounce buffer in case
a suitable application will send discontiguous IO to the given device.

Initial testing with patched FIO showed the following results (64 jobs,
128 iodepth, 1 nullb device):
IO size      READ (virt=false)   READ (virt=true)   Write (virt=false)  Write (virt=true)
----------  ------------------- -----------------  ------------------- -------------------
 1k            10.7M                8482k               10.8M              8471k
 2k            10.4M                8266k               10.4M              8271k
 4k            10.4M                8274k               10.3M              8226k
 8k            10.2M                8131k               9800k              7933k
 16k           9567k                7764k               8081k              6828k
 32k           8865k                7309k               5570k              5153k
 64k           7695k                6586k               2682k              2617k
 128k          5346k                5489k               1320k              1296k

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Link: https://lore.kernel.org/r/20210412095523.278632-1-mgurtovoy@nvidia.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>