Fix: Mark ramdisk as used in memory map

[phil-opp]

Fixes #399

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Side note: I'm not really happy with the current implementation. I think it would be better to already split the memory map given to LegacyFrameAllocator so that it uses separate region kinds for the kernel slice and the ramdisk. E.g. here:

bootloader/uefi/src/main.rs

Line 145 in 04fc88d

LegacyFrameAllocator::new(memory_map.entries().copied().map(UefiMemoryDescriptor));

However, such a change would require a lot of testing, so it's probably better to do this in a future PR.