These are usefule to specify SyncFileRange's flags argument.

Change-Id: I1726c3adf55aaebac61beed09d39a48a8e040a93
Reviewed-on: https://go-review.googlesource.com/133475
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>