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 <[e-mail]>
Cc: Josef Bacik <[e-mail]>
Cc: Yu Kuai <[e-mail]>
Cc: Markus Pargmann <[e-mail]>
Signed-off-by: Bart Van Assche <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

include/trace/events/nbd.h

···
 	),
 
 	TP_fast_assign(
-		__entry->nbd_request = 0;
+		__entry->nbd_request = NULL;
 		__entry->dev_index = index;
 		__entry->request = rq;
 	),