unix: add UMOUNT_NOFOLLOW constant

Add the UMOUNT_NOFOLLOW on Linux, available since Linux 2.6.34. See
http://man7.org/linux/man-pages/man2/umount.2.html and
https://github.com/torvalds/linux/commit/db1f05bb85d796

Fixes golang/go#18316

Change-Id: I5c7dafdbb311f287b6cf290aa8a0eaf2940b9834
Reviewed-on: https://go-review.googlesource.com/46771
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/unix/zerrors_linux_386.go b/unix/zerrors_linux_386.go
index 93383aa..02a0861 100644
--- a/unix/zerrors_linux_386.go
+++ b/unix/zerrors_linux_386.go
@@ -1755,6 +1755,7 @@
 	TUNSETVNETBE                         = 0x400454de
 	TUNSETVNETHDRSZ                      = 0x400454d8
 	TUNSETVNETLE                         = 0x400454dc
+	UMOUNT_NOFOLLOW                      = 0x8
 	VDISCARD                             = 0xd
 	VEOF                                 = 0x4
 	VEOL                                 = 0xb