dracut.sh: Fix UUID= fstab parsing in case --mount option is passed

Dracut parses /etc/fstab when --mount is option is passed (e.g. kdump).

In host_devs variable the real block device must be stored, not UUID=
There are other /etc/fstab syntax possibilities we now warn that they
are not correctly parsed. This will be fixed by another patch
when there is time to test this properly.

Signed-off-by: Thomas Renninger <trenn@suse.de>