powerpc/xmon: Use STACK_FRAME_OVERHEAD in xmon_show_stack()

We use STACK_FRAME_OVERHEAD in the exception vectors to establish
the exception frame, so it should be good enough to use here.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>