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: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>