[IA64] pgd_offset() constfication.

when compile 2.6.25-rc8-mm1, below warning happend.
because walk_page_range pass argument as "const struct mm*",
but pgd_offset() receive as "struct mm*".

  CC      mm/pagewalk.o
mm/pagewalk.c: In function 'walk_page_range':
mm/pagewalk.c:111: warning: passing argument 1 of 'pgd_offset' discards qualifiers from pointer target type
Signed-off-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NTony Luck <tony.luck@intel.com>

include/asm-ia64/pgtable.h

@@ -371,7 +371,7 @@ pgd_index (unsigned long address)
 /* The offset in the 1-level directory is given by the 3 region bits
    (61..63) and the level-1 bits.  */
 static inline pgd_t*
-pgd_offset (struct mm_struct *mm, unsigned long address)
+pgd_offset (const struct mm_struct *mm, unsigned long address)
 {
 	return mm->pgd + pgd_index(address);
 }