ublk: add UBLK_F_NO_AUTO_PART_SCAN feature flag

Add a new feature flag UBLK_F_NO_AUTO_PART_SCAN to allow users to suppress
automatic partition scanning when starting a ublk device.

This is useful for some cases in which user don't want to scan
partitions.

Users still can manually trigger partition scanning later when appropriate
using standard tools (e.g., partprobe, blockdev --rereadpt).

Reported-by: Yoav Cohen <[e-mail]>
Link: https://lore.kernel.org/linux-block/[e-mail]/
Signed-off-by: Ming Lei <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/ublk_drv.c

···
 		| UBLK_F_BUF_REG_OFF_DAEMON \
 		| (IS_ENABLED(CONFIG_BLK_DEV_INTEGRITY) ? UBLK_F_INTEGRITY : 0) \
 		| UBLK_F_SAFE_STOP_DEV \
-		| UBLK_F_BATCH_IO)
+		| UBLK_F_BATCH_IO \
+		| UBLK_F_NO_AUTO_PART_SCAN)
 
 #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \
 		| UBLK_F_USER_RECOVERY_REISSUE \
···
 
 	set_bit(UB_STATE_USED, &ub->state);
 
-	/* Schedule async partition scan for trusted daemons */
-	if (!ub->unprivileged_daemons)
-		schedule_work(&ub->partition_scan_work);
+	/* Skip partition scan if disabled by user */
+	if (ub->dev_info.flags & UBLK_F_NO_AUTO_PART_SCAN) {
+		clear_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
+	} else {
+		/* Schedule async partition scan for trusted daemons */
+		if (!ub->unprivileged_daemons)
+			schedule_work(&ub->partition_scan_work);
+	}
 
 out_put_cdev:
 	if (ret) {

include/uapi/linux/ublk_cmd.h

···
  */
 #define UBLK_F_SAFE_STOP_DEV	(1ULL << 17)
 
+/* Disable automatic partition scanning when device is started */
+#define UBLK_F_NO_AUTO_PART_SCAN (1ULL << 18)
+
 /* device state */
 #define UBLK_S_DEV_DEAD	0
 #define UBLK_S_DEV_LIVE	1