um: Fix order of dtb unflatten/early init

Scan the tree for reserved memory before unflattening it.  As Rob
Herring said:
| These should be reversed. early_init_fdt_scan_reserved_mem() works on
| the flat tree. Reserved memory needs to be reserved before
| unflatten_device_tree() starts allocating memory. Though I imagine that
| doesn't really matter for UML.

Suggested-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/lkml/YeF%2FVbxo6fAt0WLp@robh.at.kernel.org/


Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Richard Weinberger <richard@nod.at>