x86, mm: get ASLR work for hugetlb mappings

x86, mm: get ASLR work for hugetlb mappings

Matthew noticed that hugetlb doesn't participate in ASLR on x86-64.  The
reason is genereic hugetlb_get_unmapped_area() which is used on x86-64.
It doesn't support randomization and use bottom-up unmapped area lookup,
instead of usual top-down on x86-64.

x86 has arch-specific hugetlb_get_unmapped_area(), but it's used only on
x86-32.

Let's use arch-specific hugetlb_get_unmapped_area() on x86-64 too.  It
fixes the issue and make hugetlb use top-down unmapped area lookup.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@linux.intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>