Some file systems don't support sub-second time stamp resolution. Handle
these correctly by only checking nanoseconds in case Mtim.Nsec is zero.

Fixes golang/go#26034

Change-Id: I1ab400b8e09b5cfdac6b70a33f676770a48180b1
Reviewed-on: https://go-review.googlesource.com/120816
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: jimmy frasche <soapboxcicero@gmail.com>