unix: add ICMP6_FILTER on zos

Copied from golang.org/x/net/ipv6. This will allow to replace the
socket option constants in that package with constants from x/sys/unix.

Change-Id: I92a6ee9ffb4fbfcc66f1644b7677ac5bdf202367
Reviewed-on: https://go-review.googlesource.com/c/sys/+/305811
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/zerrors_zos_s390x.go

diff --git a/unix/zerrors_zos_s390x.go b/unix/zerrors_zos_s390x.go
index 4117ce0..c8c7909 100644
--- a/unix/zerrors_zos_s390x.go
+++ b/unix/zerrors_zos_s390x.go
@@ -137,6 +137,7 @@
 	IP_TTL                          = 3
 	IP_UNBLOCK_SOURCE               = 11
 	ICANON                          = 0x0010
+	ICMP6_FILTER                    = 0x26
 	ICRNL                           = 0x0002
 	IEXTEN                          = 0x0020
 	IGNBRK                          = 0x0004