dracut-module-setup.sh: skip installing driver for the loopback

interface

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2151500

Currently, kdump initrd fails to be built when dumping vmcore to
localhost via ssh or nfs,

  kdumpctl[3331]: Cannot get driver information: Operation not supported
  kdumpctl[1991]: dracut: Failed to get the driver of lo
  dracut[2020]: Failed to get the driver of lo
  kdumpctl[1775]: kdump: mkdumprd: failed to make kdump initrd
  kdumpctl[1775]: kdump: Starting kdump: [FAILED]
  systemd[1]: kdump.service: Main process exited, code=exited, status=1/FAILURE
  systemd[1]: kdump.service: Failed with result 'exit-code'.
  systemd[1]: Failed to start Crash recovery kernel arming.
  systemd[1]: kdump.service: Consumed 1.710s CPU time.

This is because the loopback interface is used for transferring vmcore and
ethtool can't get the driver of the loopback interface. In fact, once
COFNIG_NET is enabled, the loopback device is enabled and there is no driver
for the loopback device. So skip installing driver for the loopback device.
The loopback interface is implemented in linux/drivers/net/loopback.c
and always has the name "lo". So we can safely tell if a network
interface is the loopback interface by its name.

Fixes: a65dde2d ("Reduce kdump memory consumption by only installing needed NIC drivers")
Reported-by: Martin Pitt <mpitt@redhat.com>
Reported-by: Rich Megginson <rmeggins@redhat.com>
Reviewed-by: Lichen Liu <lichliu@redhat.com>
Reviewed-by: Philipp Rudo <prudo@redhat.com>
Signed-off-by: Coiby Xu <coxu@redhat.com>

dracut-module-setup.sh

@@ -387,6 +387,7 @@ kdump_install_nic_driver() {
     _drivers=()
 
     for _netif in $1; do
+        [[ $_netif == lo ]] && continue
         _driver=$(_get_nic_driver "$_netif")
         if [[ -z $_driver ]]; then
             derror "Failed to get the driver of $_netif"
@@ -404,6 +405,7 @@ kdump_install_nic_driver() {
         _drivers+=("$_driver")
     done
 
+    [[ -n ${_drivers[*]} ]] || return
     instmods "${_drivers[@]}"
 }