fix: panic on retrieving creation time in getFileStats

[semihbkgr]

It fixes two subtle issues when retrieving file created time (btime):

• Use times.Stat instead of times.Get:
  times.Get doesn’t return btime on linux platforms, as noted:

Get(FileInfo) never returns btime.

• Avoid panic when btime is unavailable:
  We need to check HasBirthTime() before calling BirthTime(), to prevent panic:

func (nobtime) BirthTime() time.Time { panic("birthtime not available") }