futex: remove the pointer math from double_unlock_hb

Impact: simplify code

I mistakenly included the pointer value ordering in the
double_unlock_hb() in my previous patch. It's only necessary
in the double_lock_hb() function. This patch removes it.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
LKML-Reference: <20090312221118.11146.68610.stgit@Aeon>
Signed-off-by: Ingo Molnar <mingo@elte.hu>