Try to install PHY and MDIO bus drivers explicitly

Resolves: https://issues.redhat.com/browse/RHEL-7028

Currently, nfs dumping fails on some machines that has a dedicated PHY
driver (dealing with the physical layer) or MDIO bus (connecting the MAC
to PHY devices) driver. This is because kexec-tools doesn't install
dedicated PHY or MDIO driver explicitly. Usually a NIC driver shouldn't
specify the dependency on the needed PHY or MDIO driver because it
shouldn't a NIC (medium access control, MAC) driver is for dealing with
the Data link layer and a PHY driver is for physical layer. So as long
as a MAC driver can talk to the PHY layer via APIs, it shouldn't care
which PHY driver or device it's talking to. So when the
dependency on a PHY driver or MDIO driver is not found by dracut's
instmods, the PHY or MDIO driver won't be installed.

This patch passes =drivers/net/phy and =drivers/net/mdio to dracut's
instmods which will only install in-use PHY or MDIO driver(s).

Note ideally we should find out which PHY driver is used by a NIC but
unfortunately currently no universal way can be found
(/sys/class/net/NIC_NAME/phydev/driver/module can be used to find the
 name of the PHY driver for some NICs but it doesn't exist for some NICs
like Qualcomm Atheros AR8031). So is it for a MDIO bus driver.
Fortunately currently no huge memory consumption is found for a PHY or
MDIO driver.

Fixes: a65dde2d ("Reduce kdump memory consumption by only installing needed NIC drivers")
Reported-by: Doreen Alongi <dalongi@redhat.com>
Signed-off-by: Coiby Xu <coxu@redhat.com>

dracut-module-setup.sh

@@ -384,7 +384,7 @@ _get_hpyerv_physical_driver() {
 kdump_install_nic_driver() {
     local _netif _driver _drivers
 
-    _drivers=()
+    _drivers=('=drivers/net/phy' '=drivers/net/mdio')
 
     for _netif in $1; do
         [[ $_netif == lo ]] && continue