xtensa: use buddy allocator for PTE table

At the moment xtensa uses slab allocator for PTE table.  It doesn't work
with enabled split page table lock: slab uses page->slab_cache and
page->first_page for its pages.  These fields share stroage with
page->ptl.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Chris Zankel <chris@zankel.net>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>