powerpc64/kexec: Hard disable ftrace before switching to the new kernel

If function_graph tracer is enabled during kexec, we see the below
exception in the simulator:
	root@(none):/# kexec -e
	kvm: exiting hardware virtualization
	kexec_core: Starting new kernel
	[   19.262020070,5] OPAL: Switch to big-endian OS
	kexec: Starting switchover sequence.
	Interrupt to 0xC000000000004380 from 0xC000000000004380
	** Execution stopped: Continuous Interrupt, Instruction caused exception,  **

Now that we have a more effective way to completely disable ftrace on
ppc64, let's also use that before switching to a new kernel during
kexec.
Signed-off-by: NNaveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/machine_kexec.c

@@ -98,12 +98,14 @@ void machine_kexec(struct kimage *image)
 	int save_ftrace_enabled;
 
 	save_ftrace_enabled = __ftrace_enabled_save();
+	this_cpu_disable_ftrace();
 
 	if (ppc_md.machine_kexec)
 		ppc_md.machine_kexec(image);
 	else
 		default_machine_kexec(image);
 
+	this_cpu_enable_ftrace();
 	__ftrace_enabled_restore(save_ftrace_enabled);
 
 	/* Fall back to normal restart if we're still alive. */