nbd: Use NULL to represent a pointer

This patch fixes the following sparse warnings:

drivers/block/nbd.c: note: in included file (through include/trace/trace_events.h, include/trace/define_trace.h, include/trace/events/nbd.h):
./include/trace/events/nbd.h:61:1: warning: Using plain integer as NULL pointer
drivers/block/nbd.c: note: in included file (through include/trace/perf.h, include/trace/define_trace.h, include/trace/events/nbd.h):
./include/trace/events/nbd.h:61:1: warning: Using plain integer as NULL pointer

Cc: Christoph Hellwig <hch@lst.de>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Yu Kuai <yukuai3@huawei.com>
Cc: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240510202313.25209-2-bvanassche@acm.org


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