futex: Update futex_wait_setup comments about locking

Reviving a cleanup I had done about a year ago as part of a larger
futex_set_wait proposal. Over the years, the locking of the hashed
futex queue got improved, so that some of the "rare but normal" race
conditions described in comments can't actually happen anymore.

Signed-off-by: Michel Lespinasse <walken@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Darren Hart <dvhltc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20110307020750.GA31188@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>