[PARISC] Fix futex support

Implements futex op support and makes futex cmpxchg atomic.
Tested on 64-bit SMP kernel running on 2 x PA8700s.

[jejb: checkpatch fixes]
Signed-off-by: Carlos O'Donell <carlos@systemhalted.org>
Tested-by: John David Anglin <dave.anglin@bell.net>
Cc: stable@kernel.org
Signed-off-by: James Bottomley <JBottomley@Parallels.com>