powerpc/kdump: Override crash_free_reserved_phys_range to avoid freeing RTAS

The crashkernel region will almost always overlap RTAS. If we free the
crashkernel region via "echo 0 > /sys/kernel/kexec_crash_size" then we will
free RTAS and the machine will crash in confusing and exciting ways.

Override crash_free_reserved_phys_range and check for overlap with RTAS.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>