[IA64] Prefetch mmap_sem in ia64_do_page_fault()

Take a hint from an x86_64 optimization by Arjan van de Ven and use it
for ia64.  See a9ba9a3b

Prefetch the mmap_sem, which is critical for the performance of the page fault
handler.

Note: mm may be NULL but I guess that is safe.
See 458f9355

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>