x86/mm/64: Do not dereference non-present PGD entries

The code for preallocate_vmalloc_pages() was written under the
assumption that the p4d_offset() and pud_offset() functions will perform
present checks before dereferencing the parent entries.

This assumption is wrong an leads to a bug in the code which causes the
physical address found in the PGD be used as a page-table page, even if
the PGD is not present.

So the code flow currently is:

	pgd = pgd_offset_k(addr);
	p4d = p4d_offset(pgd, addr);
	if (p4d_none(*p4d))
		p4d = p4d_alloc(&init_mm, pgd, addr);

This lacks a check for pgd_none() at least, the correct flow would be:

	pgd = pgd_offset_k(addr);
	if (pgd_none(*pgd))
		p4d = p4d_alloc(&init_mm, pgd, addr);
	else
		p4d = p4d_offset(pgd, addr);

But this is the same flow that the p4d_alloc() and the pud_alloc()
functions use internally, so there is no need to duplicate them.

Remove the p?d_none() checks from the function and just call into
p4d_alloc() and pud_alloc() to correctly pre-allocate the PGD entries.

Reported-and-tested-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Fixes: 6eb82f99 ("x86/mm: Pre-allocate P4D/PUD pages for vmalloc area")
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>