null_blk: replace null_process_cmd() call in null_zone_write()

As a preparation to support partial data transfer due to badblocks,
replace the null_process_cmd() call in null_zone_write() with equivalent
calls to null_handle_badblocks() and null_handle_memory_backed(). This
commit does not change behavior. It will enable null_handle_badblocks()
to return the size of partial data transfer in the following commit,
allowing null_zone_write() to move write pointers appropriately.

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20250226100613.1622564-4-shinichiro.kawasaki@wdc.com


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