dracut.sh: Fixup previous commit to only read /sys and /proc in hostonly mode

The gnome-ostree build system generates dracut initramfs images on the
build server, therefore not in hostonly mode.  The build system at the
moment doesn't mount /sys, and the previous commit caused a hard
failure due to lack of /sys/devices.

Because we only want /sys/devices in hostonly mode, just move those
bits inside the hostonly conditional above.