null_blk: do partial IO for bad blocks

The current null_blk implementation checks if any bad blocks exist in
the target blocks of each IO. If so, the IO fails and data is not
transferred for all of the IO target blocks. However, when real storage
devices have bad blocks, the devices may transfer data partially up to
the first bad blocks (e.g., SAS drives). Especially, when the IO is a
write operation, such partial IO leaves partially written data on the
device.

To simulate such partial IO using null_blk, introduce the new parameter
'badblocks_partial_io'. When this parameter is set,
null_handle_badblocks() returns the number of the sectors for the
partial IO as its third pointer argument. Pass the returned number of
sectors to the following calls to null_handle_memory_backend() in
null_process_cmd() and null_zone_write().

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20250226100613.1622564-6-shinichiro.kawasaki@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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