powerpc: Fix incorrect setting of __HAVE_ARCH_PTE_SPECIAL

[I'm going to fix upstream differently, by having all CPU types
actually support _PAGE_SPECIAL, but I prefer the simple and obvious
fix for -stable. -- Ben]

The test that decides whether to define __HAVE_ARCH_PTE_SPECIAL on
powerpc is bogus and will end up always defining it, even when
_PAGE_SPECIAL is not supported (in which case it's 0) such as on
8xx or 40x processors.
Signed-off-by: Bernhard Weirich <bernhard.weirich@riedel.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

arch/powerpc/include/asm/pte-common.h

@@ -176,7 +176,7 @@ extern unsigned long bad_call_to_PMD_PAGE_SIZE(void);
 #define HAVE_PAGE_AGP
 
 /* Advertise support for _PAGE_SPECIAL */
-#ifdef _PAGE_SPECIAL
+#if _PAGE_SPECIAL != 0
 #define __HAVE_ARCH_PTE_SPECIAL
 #endif