Use floor of mtime instead of rounding for thumb property

Use floor of mtime instead of rounding for thumb property

This seems to be the correct thing to do, at least more in line with
what at least one other implementation does.  Anything using
gnome-desktop [0] effectively does the same, as
gnome_desktop_thumbnail_is_valid applies atol(3) to mtime for
comparison and time_t on GNU/Linux is a signed int.
[0] https://git.gnome.org/browse/gnome-desktop/
* lisp/image-dired.el (image-dired-create-thumb): Use floor here.