futex: use fast_gup()

Change the get_user_pages() call with fast_gup() which doesn't require holding
the mmap_sem thereby removing the mmap_sem from all fast paths.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>