Exclude zram devices, they don't have a partition table

When zram devices are formatted and mounted, they appear in the output of lsblk. However, they don't have a partition table, so they should be excluded from the partition list.

I tried to use the --exclude 253 feature from lsblk, but this is not really clear from the kernel documentation (https://github.com/torvalds/linux/blob/master/Documentation/admin-guide/devices.txt) that it will always be that number. It seems to be a dynamic range. So I fall back on a good'ol'grep.