x86_64, ftrace: Make ftrace use kernel identity mapping to modify code

On x86_64, kernel text mappings are mapped read-only with
CONFIG_DEBUG_RODATA. So use the kernel identity mapping instead
of the kernel text mapping to modify the kernel text.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <20091029024821.080941108@sbs-t61.sc.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/mm/pageattr.c

@@ -279,8 +279,7 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
 		   __pa((unsigned long)__end_rodata) >> PAGE_SHIFT))
 		pgprot_val(forbidden) |= _PAGE_RW;
 
-#if defined(CONFIG_X86_64) && defined(CONFIG_DEBUG_RODATA) && \
-	!defined(CONFIG_DYNAMIC_FTRACE)
+#if defined(CONFIG_X86_64) && defined(CONFIG_DEBUG_RODATA)
 	/*
 	 * Once the kernel maps the text as RO (kernel_set_to_readonly is set),
 	 * kernel text mappings for the large page aligned text, rodata sections