x86: Fix fixmap page order for FIX_TEXT_POKE0,1

Masami reported:

> Since the fixmap pages are assigned higher address to lower,
> text_poke() has to use it with inverted order (FIX_TEXT_POKE1
> to FIX_TEXT_POKE0).

I prefer to just invert the order of the fixmap declaration.
It's simpler and more straightforward.

Backward fixmaps seems to be used by both x86 32 and 64.

It's really rare but a nasty bug, because it only hurts when
instructions to patch are crossing a page boundary. If this
happens, the fixmap write accesses will spill on the following
fixmap, which may very well crash the system. And this does not
crash the system, it could leave illegal instructions in place.
Thanks Masami for finding this.

It seems to have crept into the 2.6.30-rc series, so this calls
for a -stable inclusion.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Acked-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: <stable@kernel.org>
LKML-Reference: <20090701213722.GH19926@Krystal>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/fixmap.h

@@ -111,8 +111,8 @@ enum fixed_addresses {
 #ifdef CONFIG_PARAVIRT
 	FIX_PARAVIRT_BOOTMAP,
 #endif
-	FIX_TEXT_POKE0,	/* reserve 2 pages for text_poke() */
-	FIX_TEXT_POKE1,
+	FIX_TEXT_POKE1,	/* reserve 2 pages for text_poke() */
+	FIX_TEXT_POKE0, /* first page is last, because allocation is backward */
 	__end_of_permanent_fixed_addresses,
 	/*
 	 * 256 temporary boot-time mappings, used by early_ioremap(),