Configurations built with support for 52-bit virtual addressing can also
run on CPUs that only support 48 bits of VA space, in which case only
that part of swapper_pg_dir that represents the 48-bit addressable
region is relevant, and everything else is ignored by the hardware.

Our software pagetable walker has little in the way of input address
validation, and so it will happily start a walk from an address that is
not representable by the number of paging levels that are actually
active, resulting in lots of bogus output from the page table dumper
unless we take care to start at a valid address.

So define the start address at runtime based on vabits_actual.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-82-ardb+git@google.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>