selftests: ublk: correct last_rw map type in seq_io.bt

The last_rw map is initialized with a value of 0 but later assigned the
value args.sector + args.nr_sector, which has type sector_t = u64.
bpftrace complains about the type mismatch between int64 and uint64:
trace/seq_io.bt:18:3-59: ERROR: Type mismatch for @last_rw: trying to assign value of type 'uint64' when map already contains a value of type 'int64'
        @last_rw[$dev, str($2)] = (args.sector + args.nr_sector);

Cast the initial value to uint64 so bpftrace will load the program.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: calebsander

tools/testing/selftests/ublk/trace/seq_io.bt

@@ -4,7 +4,7 @@
 	$3:     strlen($2)
 */
 BEGIN {
-	@last_rw[$1, str($2)] = 0;
+	@last_rw[$1, str($2)] = (uint64)0;
 }
 tracepoint:block:block_rq_complete
 {