rootfs-block/module-setup.sh: check root_devs in cmdline_rootfs()

In case of "--no-hostonly-default-device", we do not need
the root device, thus add this check.

Also fixed the stale "root_dev" export.

Signed-off-by: Xunlei Pang <xlpang@redhat.com>

dracut.sh

@@ -1307,7 +1307,7 @@ export initdir dracutbasedir \
     dracutmodules force_add_dracutmodules add_dracutmodules omit_dracutmodules \
     mods_to_load \
     fw_dir drivers_dir debug no_kernel kernel_only \
-    omit_drivers mdadmconf lvmconf root_dev \
+    omit_drivers mdadmconf lvmconf root_devs \
     use_fstab fstab_lines libdirs fscks nofscks ro_mnt \
     stdloglvl sysloglvl fileloglvl kmsgloglvl logfile \
     debug host_fs_types host_devs swap_devs sshkey add_fstab \

modules.d/95rootfs-block/module-setup.sh

@@ -32,6 +32,12 @@ cmdline_journal() {
 cmdline_rootfs() {
     local _dev=/dev/block/$(find_root_block_device)
     local _fstype _flags _subvol
+
+    # "--no-hostonly-default-device" can result in empty root_devs
+    if [ "${#root_devs[@]}" -eq 0 ]; then
+        return
+    fi
+
     if [ -e $_dev ]; then
         printf " root=%s" "$(shorten_persistent_dev "$(get_persistent_dev "$_dev")")"
         _fstype="$(find_mp_fstype /)"