net: ipv4: avoid unused variable warning for sysctl

The newly introudced ip_min_valid_pmtu variable is only used when
CONFIG_SYSCTL is set:

net/ipv4/route.c:135:12: error: 'ip_min_valid_pmtu' defined but not used [-Werror=unused-variable]

This moves it to the other variables like it, to avoid the harmless
warning.

Fixes: c7272c2f1229 ("net: ipv4: don't allow setting net.ipv4.route.min_pmtu below 68")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -133,8 +133,6 @@ static int ip_rt_min_advmss __read_mostly	= 256;
 
 static int ip_rt_gc_timeout __read_mostly	= RT_GC_TIMEOUT;
 
-static int ip_min_valid_pmtu __read_mostly	= IPV4_MIN_MTU;
-
 /*
  *	Interface to generic destination cache.
  */
@@ -2821,6 +2819,7 @@ void ip_rt_multicast_event(struct in_device *in_dev)
 static int ip_rt_gc_interval __read_mostly  = 60 * HZ;
 static int ip_rt_gc_min_interval __read_mostly	= HZ / 2;
 static int ip_rt_gc_elasticity __read_mostly	= 8;
+static int ip_min_valid_pmtu __read_mostly	= IPV4_MIN_MTU;
 
 static int ipv4_sysctl_rtcache_flush(struct ctl_table *__ctl, int write,
 					void __user *buffer,