filesystem: make link handling more robust

filesystem: make link handling more robust

The previous patch fixed making linked protectors to /dev/root, by
setting Mount.Device to the real device node rather than /dev/root.

That's good, but it also hints that the linked protector handling is
unnecessarily fragile, as it relies on the device node name matching
exactly.  The Linux kernel allows the same device to have multiple
device nodes, and path comparisons are slow and error-prone in general.

Change it to compare the device number instead.