Fix check for available /dev/md* devices

/dev/md* might also expand devices like /dev/md/0:

| # ls -la /dev/md*
| brw-rw---- 1 root disk 9, 0 Oct 13 19:01 /dev/md0
|
| /dev/md:
| total 0
| drwxr-xr-x  2 root root   60 Oct 13 19:01 ./
| drwxr-xr-x 15 root root 3860 Oct 13 19:02 ../
| lrwxrwxrwx  1 root root    6 Oct 13 19:01 0 -> ../md0

We're interested only in /dev/md[0-9]* devices though.