The previous cherry-pick merge of fadvice from Gerrit added the
fadvise system calls, but from before the errno CL, which went in
concurrently.

Change-Id: I6f01e020bfe2930a8ea2fdbe4998ab9e2669ce14
Reviewed-on: https://go-review.googlesource.com/8199Reviewed-by: Rob Pike <r@golang.org>