ipconfig: Correctly initialise ic_nameservers

ic_nameservers, which stores the list of name servers discovered by
ipconfig, is initialised (i.e. has all of its elements set to NONE, or
0xffffffff) by ic_nameservers_predef() in the following scenarios:

 - before the "ip=" and "nfsaddrs=" kernel command line parameters are
   parsed (in ip_auto_config_setup());
 - before autoconfiguring via DHCP or BOOTP (in ic_bootp_init()), in
   order to clear any values that may have been set after parsing "ip="
   or "nfsaddrs=" and are no longer needed.

This means that ic_nameservers_predef() is not called when neither "ip="
nor "nfsaddrs=" is specified on the kernel command line. In this
scenario, every element in ic_nameservers remains set to 0x00000000,
which is indistinguishable from ANY and causes pnp_seq_show() to write
the following (bogus) information to /proc/net/pnp:

  #MANUAL
  nameserver 0.0.0.0
  nameserver 0.0.0.0
  nameserver 0.0.0.0

This is potentially problematic for systems that blindly link
/etc/resolv.conf to /proc/net/pnp.

Ensure that ic_nameservers is also initialised when neither "ip=" nor
"nfsaddrs=" are specified by calling ic_nameservers_predef() in
ip_auto_config(), but only when ip_auto_config_setup() was not called
earlier. This causes the following to be written to /proc/net/pnp, and
is consistent with what gets written when ipconfig is configured
manually but no name servers are specified on the kernel command line:

  #MANUAL
Signed-off-by: Chris Novakovic <chris@chrisn.me.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ipconfig.c

@@ -750,6 +750,11 @@ static void __init ic_bootp_init_ext(u8 *e)
  */
 static inline void __init ic_bootp_init(void)
 {
+	/* Re-initialise all name servers to NONE, in case any were set via the
+	 * "ip=" or "nfsaddrs=" kernel command line parameters: any IP addresses
+	 * specified there will already have been decoded but are no longer
+	 * needed
+	 */
 	ic_nameservers_predef();
 
 	dev_add_pack(&bootp_packet_type);
@@ -1370,6 +1375,13 @@ static int __init ip_auto_config(void)
 	int err;
 	unsigned int i;
 
+	/* Initialise all name servers to NONE (but only if the "ip=" or
+	 * "nfsaddrs=" kernel command line parameters weren't decoded, otherwise
+	 * we'll overwrite the IP addresses specified there)
+	 */
+	if (ic_set_manually == 0)
+		ic_nameservers_predef();
+
 #ifdef CONFIG_PROC_FS
 	proc_create("pnp", 0444, init_net.proc_net, &pnp_seq_fops);
 #endif /* CONFIG_PROC_FS */
@@ -1593,6 +1605,7 @@ static int __init ip_auto_config_setup(char *addrs)
 		return 1;
 	}
 
+	/* Initialise all name servers to NONE */
 	ic_nameservers_predef();
 
 	/* Parse string for static IP assignment.  */