support crtime on ext4

[dlqqq]

Right now, out-of-band edits change the modified time of the file, and thus cause a new file ID to be generated when index() is called at that path on ext* filesystems.

A huge improvement would be to leverage the statx() system call available on newer versions of Linux, and store the file creation time, which is preserved on edits. This would allow out-of-band edits to preserve the file ID on ext4 filesystems.