arch/tile: allow building Linux with transparent huge pages enabled

The change adds some infrastructure for managing tile pmd's more generally,
using pte_pmd() and pmd_pte() methods to translate pmd values to and
from ptes, since on TILEPro a pmd is really just a nested structure
holding a pgd (aka pte).  Several existing pmd methods are moved into
this framework, and a whole raft of additional pmd accessors are defined
that are used by the transparent hugepage framework.

The tile PTE now has a "client2" bit.  The bit is used to indicate a
transparent huge page is in the process of being split into subpages.

This change also fixes a generic bug where the return value of the
generic pmdp_splitting_flush() was incorrect.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

include/asm-generic/pgtable.h

@@ -158,9 +158,8 @@ static inline void pmdp_set_wrprotect(struct mm_struct *mm,
 #endif
 
 #ifndef __HAVE_ARCH_PMDP_SPLITTING_FLUSH
-extern pmd_t pmdp_splitting_flush(struct vm_area_struct *vma,
-				  unsigned long address,
-				  pmd_t *pmdp);
+extern void pmdp_splitting_flush(struct vm_area_struct *vma,
+				 unsigned long address, pmd_t *pmdp);
 #endif
 
 #ifndef __HAVE_ARCH_PTE_SAME