i386: use page allocator to allocate thread_info structure

i386 uses kmalloc to allocate the threadinfo structure assuming that the
allocations result in a page sized aligned allocation.  That has worked so
far because SLAB exempts page sized slabs from debugging and aligns them in
special ways that goes beyond the restrictions imposed by
KMALLOC_ARCH_MINALIGN valid for other slabs in the kmalloc array.

SLUB also works fine without debugging since page sized allocations neatly
align at page boundaries.  However, if debugging is switched on then SLUB
will extend the slab with debug information.  The resulting slab is not
longer of page size.  It will only be aligned following the requirements
imposed by KMALLOC_ARCH_MINALIGN.  As a result the threadinfo structure may
not be page aligned which makes i386 fail to boot with SLUB debug on.

Replace the calls to kmalloc with calls into the page allocator.

An alternate solution may be to create a custom slab cache where the
alignment is set to PAGE_SIZE.  That would allow slub debugging to be
applied to the threadinfo structure.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Cc: William Lee Irwin III <wli@holomorphy.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>