unix: add vmsockets ioctl constant on Linux

To be used e.g. in github.com/mdlayher/vsock

Change-Id: I52148a6e6c53c022c07e2cc8bf7348a96445c17b
Reviewed-on: https://go-review.googlesource.com/76193
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
diff --git a/unix/zerrors_linux_386.go b/unix/zerrors_linux_386.go
index bb8a772..8947248 100644
--- a/unix/zerrors_linux_386.go
+++ b/unix/zerrors_linux_386.go
@@ -623,6 +623,7 @@
 	IN_OPEN                              = 0x20
 	IN_Q_OVERFLOW                        = 0x4000
 	IN_UNMOUNT                           = 0x2000
+	IOCTL_VM_SOCKETS_GET_LOCAL_CID       = 0x7b9
 	IPPROTO_AH                           = 0x33
 	IPPROTO_BEETPH                       = 0x5e
 	IPPROTO_COMP                         = 0x6c