Add FALLOC_* constants from /usr/include/linux/falloc.h to use with
Fallocate (golang.org/x/sys/unix package).

Fix golang/go#10599

Change-Id: Ie001e554250bb12472c612adcb4b79c6f72cac34
Reviewed-on: https://go-review.googlesource.com/36336
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>