Update ifc_len field of struct ifconf passed for the ioctl SIOCGIFCONF32

(i.e. under COMPAT_FREEBSD32) in case ifconf() returned success to match
the native SIOCGIFCONF behavior.

PR:		kern/158369
Reported by:	Paul Procacci <pprocacci att gmail com>
MFC after:	1 week

sys/net/if.c

@@ -2467,6 +2467,8 @@ ifioctl(struct socket *so, u_long cmd, caddr_t data, struct thread *td)
 
 			error = ifconf(SIOCGIFCONF, (void *)&ifc);
 			CURVNET_RESTORE();
+			if (error == 0)
+				ifc32->ifc_len = ifc.ifc_len;
 			return (error);
 		}
 #endif