futex: rely on get_user_pages() for shared futexes

On the way of getting rid of the mmap_sem requirement for shared futexes,
start by relying on get_user_pages().

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>