futex: update futex commentary

Impact: cleanup

The futex_hash_bucket can be a bit confusing when first looking
at the code as it is a shared queue (and futex_q isn't a queue
at all, but rather an element on the queue).

The mmap_sem is no longer held outside of the
futex_handle_fault() routine, yet numerous comments refer to it.
The fshared argument is no an integer.  I left some of these
comments along as they are simply removed in future patches.

Some of the commentary refering to futexes by virtual page
mappings was not very clear, and completely accurate (as for
shared futexes both the page and the offset are used to
determine the key).  For the purposes of the function
description, just referring to "the futex" seems sufficient.

With hashed futexes we now access the page after the hash-bucket
is locked, and not only after it is enqueued.

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: <20090312075537.9856.29954.stgit@Aeon>
Signed-off-by: Ingo Molnar <mingo@elte.hu>