parisc: Fix validity check of pointer size argument in new CAS implementation

As noted by Christoph Biedl, passing a pointer size of 4 in the new CAS
implementation causes a kernel crash.  The attached patch corrects the
off by one error in the argument validity check.

In reviewing the code, I noticed that we only perform word operations
with the pointer size argument.  The subi instruction intentionally uses
a word condition on 64-bit kernels.  Nullification was used instead of a
cmpib instruction as the branch should never be taken.  The shlw
pseudo-operation generates a depw,z instruction and it clears the target
before doing a shift left word deposit.  Thus, we don't need to clip the
upper 32 bits of this argument on 64-bit kernels.

Tested with a gcc testsuite run with a 64-bit kernel.  The gcc atomic
code in libgcc is the only direct user of the new CAS implementation
that I am aware of.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Cc: stable@vger.kernel.org # 3.13+
Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/kernel/syscall.S

@@ -690,15 +690,15 @@ cas_action:
 	/* ELF32 Process entry path */
 lws_compare_and_swap_2:
 #ifdef CONFIG_64BIT
-	/* Clip the input registers */
+	/* Clip the input registers. We don't need to clip %r23 as we
+	   only use it for word operations */
 	depdi	0, 31, 32, %r26
 	depdi	0, 31, 32, %r25
 	depdi	0, 31, 32, %r24
-	depdi	0, 31, 32, %r23
 #endif
 
 	/* Check the validity of the size pointer */
-	subi,>>= 4, %r23, %r0
+	subi,>>= 3, %r23, %r0
 	b,n	lws_exit_nosys
 
 	/* Jump to the functions which will load the old and new values into