efi_loader: Keep memory mapping sorted when splitting an entry

The code assumes sorted mappings in descending address order. When
splitting a mapping, insert the new part next to the current mapping.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>