Futimes() only takes microsecond resolution while the FUSE call
Utimens() wants nanosecond precision.
This is why UTIME_OMIT did not work - this change fixes the
xfstests generic/258 test failure.
The go library does not provide a FutimesNano() function which is
why I use UtimesNano() on /proc/self/fd/n.
This is what the Go library does in Futimes().