ipv4: clarify the range of broken freebsd kernel versions

Change-Id: Ic37732f28e747c98cb513e100983d685efd3928d
Reviewed-on: https://go-review.googlesource.com/c/net/+/173077
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

ipv4/helper.go

@@ -32,7 +32,7 @@ var (
 // See golang.org/issue/30899.
 func adjustFreeBSD32(m *socket.Message) {
 	// FreeBSD 12.0-RELEASE is affected by https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236737
-	if freebsdVersion >= 1200086 && freebsdVersion < 1201000 {
+	if 1200086 <= freebsdVersion && freebsdVersion < 1201000 {
 		l := (m.NN + 4 - 1) &^ (4 - 1)
 		if m.NN < l && l <= len(m.OOB) {
 			m.NN = l