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Currently memblock_reserve() or memblock_free() don't handle overlaps of
any kind.  There is some special casing for coalescing exactly adjacent
regions but that's about it.

This is annoying because typically memblock_reserve() is used to mark
regions passed by the firmware as reserved and we all know how much we can
trust our firmwares...

Also, with the current code, if we do something it doesn't handle right
such as trying to memblock_reserve() a large range spanning multiple
existing smaller reserved regions for example, or doing overlapping
reservations, it can silently corrupt the internal region array, causing
odd errors much later on, such as allocations returning reserved regions
etc...

This patch rewrites the underlying functions that add or remove a region
to the arrays.  The new code is a lot more robust as it fully handles
overlapping regions.  It's also, imho, simpler than the previous
implementation.

In addition, while doing so, I ...