x86/kvm: Fix pvclock vsyscall fixmap

The physical memory fixmapped for the pvclock clock_gettime vsyscall
was allocated, and thus is not a kernel symbol. __pa() is the proper
method to use in this case.

Fixes the crash below when booting a next-20130204+ smp guest on a
3.8-rc5+ KVM host.

[    0.666410] udevd[97]: starting version 175
[    0.674043] udevd[97]: udevd:[97]: segfault at ffffffffff5fd020
     ip 00007fff069e277f sp 00007fff068c9ef8 error d
Acked-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGleb Natapov <gleb@redhat.com>

arch/x86/kernel/pvclock.c

@@ -185,7 +185,7 @@ int __init pvclock_init_vsyscall(struct pvclock_vsyscall_time_info *i,
 
 	for (idx = 0; idx <= (PVCLOCK_FIXMAP_END-PVCLOCK_FIXMAP_BEGIN); idx++) {
 		__set_fixmap(PVCLOCK_FIXMAP_BEGIN + idx,
-			     __pa_symbol(i) + (idx*PAGE_SIZE),
+			     __pa(i) + (idx*PAGE_SIZE),
 			     PAGE_KERNEL_VVAR);
 	}