arm64/mm: Remove randomization of the linear map

Since commit

  97d6786e0669 ("arm64: mm: account for hotplug memory when randomizing the linear region")

the decision whether or not to randomize the placement of the system's
DRAM inside the linear map is based on the capabilities of the CPU
rather than how much memory is present at boot time. This change was
necessary because memory hotplug may result in DRAM appearing in places
that are not covered by the linear region at all (and therefore
unusable) if the decision is solely based on the memory map at boot.

In the Android GKI kernel, which requires support for memory hotplug,
and is built with a reduced virtual address space of only 39 bits wide,
randomization of the linear map never happens in practice as a result.
And even on arm64 kernels built with support for 48 bit virtual
addressing, the wider PArange of recent CPUs means that linear map
randomization is slowly becoming a feature that only works on systems
that will soon be obsolete.

So let's just remove this feature. We can always bring it back in an
improved form if there is a real need for it.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Kees Cook <kees@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250318134949.3194334-2-ardb+git@google.com
Signed-off-by: Will Deacon <will@kernel.org>

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