futex: comment requeue key reference semantics

We've tripped over the futex_requeue drop_count refering to key2
instead of key1.  The code is actually correct, but is non-intuitive.
This patch adds an explicit comment explaining the requeue.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>