memblock: reset memblock.reserved to system init state to prevent UAF

A call to memblock_free() or memblock_phys_free() issued after memblock
data is discarded will result in use after free in
memblock_isolate_range().

When CONFIG_KASAN is enabled, this will cause a panic early in boot.
Without CONFIG_KASAN, there is a chance that memblock_isolate_range() might
scribble on memory that is now in use by somebody else.

Avoid those issues by making sure that memblock_discard points
memblock.reserved.regions back at the static buffer.

If memblock_free() or memblock_phys_free() is called after memblock memory
is discarded, that will print a warning in memblock_remove_region().
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