[IA64] __copy_user breaks on unaligned src

memcpy_mck.S::__copy_user breaks in the prefetch code under these conditions :-

* src is unaligned and
* dst is near the end of a page and
* the page after dst is unmapped.

Signed-off-by: Keith Owens <kaos@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>