block: add ioctl to read the disk sequence number

Add a new BLKGETDISKSEQ ioctl which retrieves the disk sequence number
from the genhd structure.

# ./getdiskseq /dev/loop*
/dev/loop0: 13
/dev/loop0p1: 13
/dev/loop0p2: 13
/dev/loop0p3: 13
/dev/loop1: 14
/dev/loop1p1: 14
/dev/loop1p2: 14
/dev/loop2: 5
/dev/loop3: 6

Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Matteo Croce <[e-mail]>
Tested-by: Luca Boccassi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

block/ioctl.c

···
 				BLKDEV_DISCARD_SECURE);
 	case BLKZEROOUT:
 		return blk_ioctl_zeroout(bdev, mode, arg);
+	case BLKGETDISKSEQ:
+		return put_u64(argp, bdev->bd_disk->diskseq);
 	case BLKREPORTZONE:
 		return blkdev_report_zones_ioctl(bdev, mode, cmd, arg);
 	case BLKRESETZONE:

include/uapi/linux/fs.h

···
 #define BLKSECDISCARD _IO(0x12,125)
 #define BLKROTATIONAL _IO(0x12,126)
 #define BLKZEROOUT _IO(0x12,127)
+#define BLKGETDISKSEQ _IOR(0x12,128,__u64)
 /*
  * A jump here: 130-136 are reserved for zoned block devices
  * (see uapi/linux/blkzoned.h)