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/46771Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>