PARISC futex: special case cmpxchg NULL in kernel space

upstream commit: c20a84c9

commit f9e77acd
Author: Thomas Gleixner <tglx@linutronix.de>
Date:   Sun Feb 24 02:10:05 2008 +0000

    futex: runtime enable pi and robust functionality
 

which was backported to stable based on mainline Commit
a0c1e907 added code to futex.c
to detect whether futex_atomic_cmpxchg_inatomic was implemented at run
time:

+       curval = cmpxchg_futex_value_locked(NULL, 0, 0);
+       if (curval == -EFAULT)
+               futex_cmpxchg_enabled = 1;

This is bogus on parisc, since page zero in kernel virtual space is the
gateway page for syscall entry, and should not be read from the kernel.
(That, and we really don't like the kernel faulting on its own address
 space...)

Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>