mount: Fixup phys/virt devices dumping for ghost files

When dumping a ghost file we put real device in its header,
not the (btrfs) virtual one. This is done since we put real
devices into fsnotify images (we get them from proc). That
said on fsnotify ghost restore we don't need to do path
resolution, just devices compare.

And one more thing. When dumping device for ghost file for
_non_ btrfs case we have to convert stat dev_t into kernel
dev_t as all the other places in criu manipulate the latter
ones.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>