mm/hugetlb: fix NULL-pointer dereference on 5-level paging machine

mm/hugetlb: fix NULL-pointer dereference on 5-level paging machine

I made a mistake during converting hugetlb code to 5-level paging: in
huge_pte_alloc() we have to use p4d_alloc(), not p4d_offset().

Otherwise it leads to crash -- NULL-pointer dereference in pud_alloc()
if p4d table is not yet allocated.

It only can happen in 5-level paging mode.  In 4-level paging mode
p4d_offset() always returns pgd, so we are fine.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: c2febafc6773 ("mm: convert generic code to 5-level paging")
Signed-off-by: Kirill A. Shutemov <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: <[email protected]>	[4.11+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>