[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: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>