locking/mutexes: Correct documentation on mutex optimistic spinning

The mutex optimistic spinning documentation states that we spin for
acquisition when we find that there are no pending waiters. However,
in actuality, whether or not there are waiters for the mutex doesn't
determine if we will spin for it.

This patch removes that statement and also adds a comment which
mentions that we spin for the mutex while we don't need to reschedule.

Signed-off-by: Jason Low <jason.low2@hp.com>
Acked-by: Davidlohr Bueso <davidlohr@hp.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: akpm@linux-foundation.org
Cc: tim.c.chen@linux.intel.com
Cc: paulmck@linux.vnet.ibm.com
Cc: rostedt@goodmis.org
Cc: Waiman.Long@hp.com
Cc: scott.norton@hp.com
Cc: aswin@hp.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1402511843-4721-2-git-send-email-jason.low2@hp.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>