check-disk-health.sh: Check only disks

Suppose we have these device:

    NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
    sda      8:0    0 931.5G  0 disk
    ├─sda1   8:1    0 914.4G  0 part /
    ├─sda2   8:2    0     1K  0 part
    └─sda5   8:5    0  17.1G  0 part [SWAP]
    sr0     11:0    1  1024M  0 rom

The previous implementation captures sda and also sr0, which is
not a disk.
Merge pull request #351 from maoe/fix-check-disk-health

check-disk-health: Check only disks