Don't use direct mapping to patch kernel text

From: Jan Beulich <jbeulich@novell.com>

It took me quite a while to figure out that this was the reason the Xen 
kernel (with this merged into it) doesn't boot anymore (because it sets up 
the direct mapping portion representing the kernel image as read-only), 
and the alternative code needs less 'black magic':

                if (instr >= (u8 *)VSYSCALL_START && instr < (u8*)VSYSCALL_END)
                        instr -= VSYSCALL_START - (unsigned long)&__vsyscall_0;

[AK: not strictly needed as a mainline bugfix, but let's do the Xen
people a favour]

Signed-off-by: Andi Kleen <ak@suse.de>

Index: linux/arch/x86_64/kernel/setup.c
===================================================================
--- linux.orig/arch/x86_64/kernel/setup.c
+++ linux/arch/x86_64/kernel/setup.c
@@ -498,7 +498,7 @@ void apply_alternatives(void *start, voi
 		instr = a->instr;
 		/* vsyscall code is not mapped yet. resolve it manually. */
 		if (instr >= (u8 *)VSYSCALL_START && instr < (u8*)VSYSCALL_END)
-			instr = __va(instr - (u8*)VSYSCALL_START + (u8*)__pa_symbol(&__vsyscall_0));
+			instr -= VSYSCALL_START - (unsigned long)&__vsyscall_0;
 		__inline_memcpy(instr, a->replacement, a->replacementlen);
 		diff = a->instrlen - a->replacementlen;