udev: add /dev/disk/by-diskseq symlink for block devices

udev: add /dev/disk/by-diskseq symlink for block devices

This adds another symlink for block devices:

    /dev/disk/by-diskseq/<number>

where the number is the diskseq number as exposed by the kernel. It's
useful for apps because they can use it to open a device by diskseq, in
a way that is safe against device node reuse. I.e. if a device node path
like this is passed to an app it could open the device node via the
symlink and also parse the diskseq from the path. Once the device is
opened it could compare the parsed diskseq with the one returned by
BLKGETDISKSEQ on the open node, and if it matches they know they are
talking to the right device.

Fixes: #22906