Syncfs is Linux-specific system call to synchronize to disk one
filesystem specified by fd to any of its files.

See http://man7.org/linux/man-pages/man2/syncfs.2.html for details.

Change-Id: Ia2ad6208c3fa0e9c40d5112ce938edd5d374131d
Reviewed-on: https://go-review.googlesource.com/51510Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>