x86/boot: Defer initialization of VM space related global variables

The global pseudo-constants 'page_offset_base', 'vmalloc_base' and
'vmemmap_base' are not used extremely early during the boot, and cannot be
used safely until after the KASLR memory randomization code in
kernel_randomize_memory() executes, which may update their values.

So there is no point in setting these variables extremely early, and it
can wait until after the kernel itself is mapped and running from its
permanent virtual mapping.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250513111157.717727-9-ardb+git@google.com