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 <hch@lst.de>
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Tested-by: Luca Boccassi <bluca@debian.org>
Link: https://lore.kernel.org/r/20210712230530.29323-4-mcroce@linux.microsoft.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/ioctl.c

@@ -469,6 +469,8 @@ static int blkdev_common_ioctl(struct block_device *bdev, fmode_t mode,
 				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

@@ -184,6 +184,7 @@ struct fsxattr {
 #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)