ipv4: Fix 32-bit build.

   net/ipv4/af_inet.c: In function 'snmp_get_cpu_field64':
>> net/ipv4/af_inet.c:1486:26: error: 'offt' undeclared (first use in this function)
      v = *(((u64 *)bhptr) + offt);
                             ^
   net/ipv4/af_inet.c:1486:26: note: each undeclared identifier is reported only once for each function it appears in
   net/ipv4/af_inet.c: In function 'snmp_fold_field64':
>> net/ipv4/af_inet.c:1499:39: error: 'offct' undeclared (first use in this function)
      res += snmp_get_cpu_field(mib, cpu, offct, syncp_offset);
                                          ^
>> net/ipv4/af_inet.c:1499:10: error: too many arguments to function 'snmp_get_cpu_field'
      res += snmp_get_cpu_field(mib, cpu, offct, syncp_offset);
             ^
   net/ipv4/af_inet.c:1455:5: note: declared here
    u64 snmp_get_cpu_field(void __percpu *mib, int cpu, int offt)
        ^

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>