selftests: ublk: increase max buffer size to 1MB

Increase max buffer size to 1MB, and 64KB is too small to evaluate
performance with builtin ublk server implementation.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250322093218.431419-4-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

tools/testing/selftests/ublk/kublk.h

@@ -40,7 +40,7 @@
 /* queue idle timeout */
 #define UBLKSRV_IO_IDLE_SECS		20
 
-#define UBLK_IO_MAX_BYTES               65536
+#define UBLK_IO_MAX_BYTES               (1 << 20)
 #define UBLK_MAX_QUEUES                 4
 #define UBLK_QUEUE_DEPTH                128