[PATCH] powerpc: Don't use kmalloc() for kernel stacks

In readiness for 64k pages, when THREAD_SIZE will be less than
PAGE_SIZE, ppc64 uses kmalloc() rather than __get_free_pages() to
allocate kernel stacks, and since thread_info.h was merged, so does
ppc32.  However that adds some overhead which we don't really want
when PAGE_SIZE <= THREAD_SIZE (including all ppc32 machines), so this
patch avoids it.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>