Skip uid and gid changes if request uid and gid are zero
mongodb lock file, and many interim files, has empty uid and gid
I0724 10:05:51 93643 filehandle.go:173] /db/diagnostic.data/metrics.interim fh 1333342842031408359 flush Flush [ID=0x3 Node=0x50 Uid=0 Gid=0 Pid=178] 0x10 fl=0x0 lk=0x0