[PARISC] fix panic on prefetch(NULL) on PA7300LC

Due to an errata, the PA7300LC generates a TLB miss interruption even on the
prefetch instruction.  This means that prefetch(NULL), which is supposed to be
a nop on linux actually generates a NULL deref fault.  Fix this by testing the
address of prefetch against NULL before doing the prefetch.

Cc: stable@vger.kernel.org
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

arch/parisc/include/asm/prefetch.h

@@ -21,7 +21,12 @@
 #define ARCH_HAS_PREFETCH
 static inline void prefetch(const void *addr)
 {
-	__asm__("ldw 0(%0), %%r0" : : "r" (addr));
+	__asm__(
+#ifndef CONFIG_PA20
+		/* Need to avoid prefetch of NULL on PA7300LC */
+		"	extrw,u,= %0,31,32,%%r0\n"
+#endif
+		"	ldw 0(%0), %%r0" : : "r" (addr));
 }
 
 /* LDD is a PA2.0 addition. */